Yeah yeah yeah. You got me. I've been distracted by other things and now suddenly this project is back in sight of my red eye of Mordor.
I had certainly looked at all the links, and thought I was a little afraid to tax my A500 by giving power via the bus. The GVP is an earlier model (it looks beefier than the series II) and has a big honkin' old SCSI spindle drive in it...
So, as Mech suggested, I bought an old A500 power supply, and wanted to mod it. I now know the pinouts of the GVP... they don't use the -5 and -12 volt rails...
If I use the old A500 power supply and have nothing connected to the -5 and -12V rails, that should be ok, yes?