I tried various methods like converting .dmg to .cdr (CD Master) in Disk Utility, but the resulting CD always mounted as HFS+.
Finally I googled a nice method:
hdiutil makehybrid -iso -o MyImage.iso /Volumes/SomeFolder/
will create an ISO filesystem, which can be burnt with Disk Utility and shows up like that in the Finder. That is - well, I am pretty sure - readable on PCs.
Alternatively I may use
hdiutil burn MyImage.iso
In retrospect, some of my burn products may have ended up as PC-readable too, since hybrid filesystems may have been created. I'll test them on a PC tomorrow for sure.