The definition of Tartan from the Scottish Tartans Authority:

Tartan is woven from threads which cross at right angles and the pattern - called the SETT - therefore has, of necessity, to be of rectangular format. It comprises a series of stripes which, although exceptions are not uncommon, generally are (a) the same in both warp and weft of the cloth and (b) are expressed as a half-sett which repeats, reversing as it goes, along and across the cloth, so that each half-sett is the mirror-image of its neighbour; these introduce further inviolable rules.

Please note that no where does it mention naming or registering a pattern to qualify as being a Tartan.

Please also note that no where does it say Twill Weave.

The words "Fashion Tartan" seem to have picked up a negative connotation but they are Tartan neverless.
If a Tartan is not named then please feel free to call it whatever you want.
As long as you don't try to name your Fashion Tartan using a name of a Registered Tartan or call your Tartan something stupid I think it will get by the Tartan Cops.

I have fun with the ladies at my local fabric store by going in and looking at the bolts of Fashion Tartan and in a loud voice saying "OH look honey, they have the Steve Tartan."