@Theetjuh Thanks for pointing this out and putting in the lines to comment on here! Matheos96 mentioned the comment workaround in the GitHub thread, too, but I was too hesitant to try.
Anyway, after two hours of compiling and text happily flashing over the second monitor, PuTTY decided to abort my session. 8[ I guess that’s how you learn to use use screen for everything longer than a couple of minutes. Next try tomorrow. Just wanted to throw in a thank you and letting you know it works in principle.
€dit: Next try threw an error message about corrupted memory after ~ 30 minutes. Hadn’t activated swap. Activated and tried again. Next error message after two hours unable to install binary for sdl2. Gave up exasperated.
Went back to the original plan. Wiped everything, installed the image from before the Stretch update, installed RetroPie, then updated OSMC with January patch, and finally applied danyeaw’s workaround.
Everything working fine now. With the RPi3 I can even enable the PS1 graphics enhancement without slowdown and enjoy Wipeout 2097 and Need for Speed High Stakes / Road Challenge with some crispy fresh pixels, yay. 