I use the straight stitch setting on my Janome 7700 quite often - absolutely when FMQ. The 7700 has that funky dual-function APC (automatic plate conversion) plate. When you move between straight stitch and full-width settings, the machine reminds you to check the bobbin thread first, so I imagine it can tell which setting the plate is in.
I've used the straight stitch plate a couple of times on my Elna 760 and I'm pretty sure it has a sensor to tell it which plate is installed so it won't let you select a zigzag or move the needle if the straight stitch plate is in place.