For unintentional radiators you want to try and shove yourself (I think) into "§15.103: Exempted devices". In my experience we mostly work with "digital device utilized exclusively in any transportation vehicle including motor vehicles and aircraft". However, the "certificate of conformity" comes in the form of a slew of regulatory tests not required for FCC, but required for a different agency.

Looks like the stuff you're doing is not commercial (i.e. you're not making money or commercializing it), and may perhaps fall under "§15.23: Home-built devices" if you claim that each one you modify is unique in of by itself.

I have not worked creating part-15 devices outside my normal work (where we normally are exempt because of all the additional regulatory testing done anyways); but I'm always interested in hearing how other people approach it.

Link to a USB hub declaration of conformity example: http://cache-www.belkin.com/support/dl/f5u111.pdf

A couple of useful links:
http://www.arrl.org/part-15-radio-frequency-devices (look into the Verification section)
http://www.part15.us/ (a place for questions)
http://transition.fcc.gov/Bureaus/En...2/oet62rev.pdf (short brief from FCC)