Has anyone tried any of RIP's specifically designed for 12 ink printers?

Proofing Solution's EFI or Ergosoft's StudioPrint
currently support the Canon IPF.

Do they actually treat it as a 12-color device, or do they treat it as an RGB device? I would suspect the latter...

EFI drives it as a 12 color device. You have the option of treating it as a CMYK device however this means that none of the RGB inks are used, and I believe your screening options are also limited. It is also a bit tricky because the RIP has to work in conjunction with the Media Presets selected in the printer.

Julian