Respected friend:

That's exactly my position. I have neither reason nor desire to bug people about minutiae. I don't even bother people who have already made something I know is hopelessly OOP, unless they directly ask my opinion. I concentrate on the people who are still in the research phase, and steer them away from the vast sea of misinformation as best I may. That way, someday, there will be more people out there who do know what's documentable, and they will go out and teach still others.
Nobody thinks US Army sabers are "close enough" to scimitars any more. Teaching did that. Nobody thinks people in post-roman Europe were eating stuffed pumpkin any more. Teaching did that. That's why I like teaching.

In service- Honour/Una/Alizaundre

And yet, people think that Kilts are period(They're not), and believe erroneously that family-colored tartans are period(Most definitely not). So I wonder why we're so tolerant of those? Aside from the fact that the men that support them do so because they enjoy being publicly kilt-checked at family events, and the women that support them enjoy doing the checking? The difference is, with materials available, the L-shaped chair could have easily been made almost anywhere, a 9 yard perfect plaid kilt, not so much, especially not during the eras wherein fabric was a very, very expensive commodity(Look at period dance, it's designed around the principal of showing off how much fabric you're wearing, and how much of it bears colorful dyes, a notation of one's personal wealth. You're not going to have massive amounts of blue, green, and black dye available to every person in a specific family, especially considering the processes that went into making certain darker blue dyes.)

That having been said, wood is pretty much anywhere, and while someone could relatively easily produce a stool from a treestump, or any number of other things, this creates a weight problem. And yes, directors chairs and certain kinds of stools could be used, in theory, if the person in question isn't incredibly large, or burdened with a lot of armor. Additionally, comfort wasn't always the point of this, as documentation does exist of a Watchman's chair, designed so that it Wasn't comfortable so that night shift guards couldn't easily fall asleep in it.

There is a problem with your analogy. While the modern kilt and tartans are not within SCA time frame, the belted plaid is. George Buchanan had noted in 1582 that Highlanders "...delight in varigated garments, especially stripes, and their favorite colors are purple and blue. Their ancestors wore plaids of many colors, and numbers still retain this custom...". He also noted "... in these [plaids], wrapped, not coverd...".

There is also evidence of Tartan (as opposed to plaid which is is documented to prehistoric times) in the Falkirk sett dating to 3rd C CE. While this pattern is not the bright colors of later dates, it does show that particular patterns were being used.

Martin Martin, writing c1695 in Description of the Western Isles, "The plad worn only be the men ...consists of divers colours, so as to be agreable to the nicest fancy... Every isle differs from each other in their fancy of making plads as to the stripes in breadth and colours. The humour is as different through the mainland of the highlands, in so far that they who have see those places are able, at the first view of a man's plad, to guess the place of residence."

This shows that regional tartis, as opposed to family, were in used during the latter part of the SCA period. While this is supposition, it is safe as it would take a fairly long time for people living in issolated areas to have their patterns recognizable by people from the outside.

So, while the feileadh beag is not within our time frame as well as most of the clan tartans, the feileagh mor is. We tolerate the tartan for the reason that most of the regional tartans that could be used are no longer manufactured and some no longer known.

It is also were the term "creative anachronism" comes into play. Most of us try to get as close as possible to being historically accurate. But due to many and varied reasons there are a lot of people who can't get any closer that using a kilt with modern tartan to play the role of a late period Scot. This could be the reason many people continue to use the L-chair - it is chep and easy to make, easy to transport and fighter in armor can sit in it without damaging the piece of furniture.

Sorry for the length of this.

Actually, we have a large number of patterns the people of medieval and renaissance Scotland would have called "tartan"- the word translates into English as "fabric" after all; but these were not the pathetic Sobieski Stuart tartans. They were what clothiers would now call windowpane checks. We also have proof of quite startlingly modern plaid in use in medieval Spain, where it was a sign of wealth to not only have these more complex, more expensive fabrics, but to add a good thick layer of gold on the lily by using them on the diagonal to make skin-tight cotehardie type garments.
Also, there are an entire set of surviving sett sticks that were found in the wall of a weaver's workroom, believed to date from the 1560s to the early 1700s (they were found before radiocarbon dating, and I guess nobody has bothered to do it since) that are most emphatically within the range of the simpler, more basic plaids- not the modern tartans, which evidence suggests the Sobieskis copied from plaids out of Madras, India, but similar to some "ancient" or "hunting" plaids.
If you google my modern name you will find my explanation of how clan tartans were probably made and used in our era. My information comes from my long experience as a weaver, and is based on the exigences of weaving rather than modern extrapolation. ...But if we're shifting to plaids we ought to rename this thread from here out...