I don't know if there are many old Handspring Visor fans out there, but I wrote a simple C extension that can be called from Plua2 to access the Springboard address space. So far this lets me read individual bytes off my flash backup cards, but it should enable you to read and write to any home-made hardware that you hook up. I'll try uploading the extension and some sample Lua code to the Files area here, if anyone is interested.
My main interest is getting one of the wireless SDIO cards from PalmOne or SanDisk to work on my Visors. If anyone is at all knowledgable about these, please let me know.