Or I could just try it and see what happens. It works fine.
In fact, it was quite interesting. I put a R1 disc in one drive, let it start playing, then swapped it into my second drive (which is set as R4). The DVD player asked whether I wanted to start at the beginning of the disc, or jump to where I stopped it. I chose the resume, and it played! So it appears that the region check is stored at the beginning of the disc, and it's not checked if you resume.