Since you are going to need a DER to approve the installation of your equipment in US operated aircraft, you should pick one who has access to the aircraft you are targeting. I work for a DAR in Canada, and we do this sort of thing all the time, especially King Air's, Navajo's, etc., so we already have access to the info for all the aircraft you mentioned above.
It sounds like your workstation is going to get larger. Do you expect it to span the whole cabin? This would be extremely difficult to approve, as the workstation would block access to the rear exits of the aircraft, and prevent fire-fighting there, too.
Steven Fahey, CET