> your right click method only offers 'poly image' not polyimagepro,
Yes, that's correct actually.
> I tried to add/edit the type choosing image, raster, poly image but no dice sadly.
It seems to work fine here, but I only have a few .pspimage files from a very old version of Paintshop Pro. I suspect that Poly Image doesn't support your newer version.
> If the sagethumbs has some sort of plugin for windows explorer is it possible to get tp10 to use that?
No, I don't think so.
Have you checked if PSP functions as an OLE server? It used to in the past, so there's a good chance that it still does. In the file type definition in ThumbsPlus, change Load Using to OLE and see if there's something PSP-related in the dropdown list to its right. If there are multiple options, you can try them all (well, one by one).
If that works, the quickest way to generate those hundreds of thumbnails is to launch PSP and to keep it running in the background whilst you thumbnail those images in ThumbsPlus. That way, PSP doesn't have to be re-launched for each thumbnail.