OK, installed, rebooted, looking at a nice grub> prompt now, probably because I had grub install to /dev/sda1 (changed it from MBR to boot sector as I've done up to SL5.1) so gonna try the install-with-patch one more time, this time leaving it to installing grub on /dev/sda.
Not a fscking clue as to why that should make a difference, google didn't really help me get anywhere and as the Sabayon Forum Search simply ignores 75% of the keywords, I have no idea where to look for answers.
(Glad I never touched sdb, can still boot into SL4 by changing disk order in BIOS, and yes, I know, I could create an entry in grub on sdb for SL5.3 but right now I don't even cáre to find out how. Besides, the install itself is a lot faster than learning grub and tbh I'm a little tired (of learning, but also just tired) right now.)
If it works, I'll mark this as solved.
If it doesn't, I'm gonna bin this dvd and try re-installing 5.1...
Handtekening? Zeg...ik ga niet op m'n monitor schrijven!