Here's an interesting twist.
As Iskovlun suggested, the LSL6 terp doesn't do ShowMovie. So I dropped in all the files that came with KQ6CD -- not just SIERRA.EXE, but all the drivers and other such files too, and the config file for good measure. Given two testing videos -- FS1 and my THEEND -- I get a pretty weird result. It's all tiny and staggering. Whereas if I do the opposite, replacing KQ6's FS1 with THEEND, I get a color problem.
Neither of these things happen in ScummVM, be they Template or KQ6. The part where it takes so freakin' long to get FS1 to play in KQ6 doesn't make testing for fixes any easier.