CASH MONEY if you can show me a machine that will cut a digitially printed, laminated decal perfectly at 12'.... Unlesss you're track fed, it ain't happening. Too many variables, starting with just stretching!
You can get there, but it is a major pain in the neck. As you mentioned, media elongation occurs when laminating. (plus your printer won't print 12 feet exactly; scale variations between the printer and cutter are a given)
So you gotta measure what 12 feet is, exactly, after lamination. Then use that variable to calibrate the cutter's x-axis tracking distance using the thickness of the laminated print as the basis of the tracking length. Thicker materials track further. We all know that from the first time we cut a drop-shadow in cast and the primary element in calendered, or vice versa. (they do not match up)
If you do all this, then yeah, you can go long, accurately. But it's a pain. Thus, I'd advise...
If it's a decal that's 12 feet, hand cut it. It'll be way faster.
If it's a bunch of decals going out 12 feet, split it into 2 or 3 cut jobs. Easy, quick and accurate.
My $0.02,
Jim