I seem to have success, though I'm not sure why.
After posting last I went back and tried an install of 5.2 - went in perfectly first time with the earlier installer from the live desktop.
I then re-attempted installing 5.4. First try the cursor showed that the software was trying to run, and I got the bouncing CD animation, which eventually stopped with no GUI. I tried it a second time, and as I'd observed a few times before, it started this time, but got stuck trying to find the hard drives (gave up after 10 min).
So I opened up Yakuake and started the liveinstaller from there, which (slowly) took me through the full GUI install. I told it to replace the existing linux system on sda, and it formatted the partitions and installed fine. I now have a working Sabayon 5.4 system.
So thanks for your help on this. I'd guess that the problem is 'fixed' though not exactly solved, but that doesn't matter in a sense because the next version will be along soon with it's own characteristics. I really like the KDE 5.4 desktop, and have added some of my preferred tools (DigiKam, Opera, Thunderbird, Audacity) and it's crisp and responsive.