I operate a TNC-x on the Raspberry that I use with other computers so your rational is sound.
You have to plug the TNC-x in to either a powered hub or directly into the Pi. The Pi site would warn you away from a direct connection but the TNC-x draws very little power. My system is built for mobile/field day/emergency use. The tnc-pi makes a very neat and tidy package. The TNC-x comes from the factory with Audio connectors. I've used a drill and a file to fit it with a DB9 to make it compatible with the Kantronix connector in place of the upper audio jack and left the Audio In jack (speaker from radio) in place to make it work readily with Mobile radio's. MFJ makes a TNC/radio box (they are $50 new) but can be had for $20 used on Ebay. They take some rework to accept the newer radio mic plugs (either 6 pin phone, or cat-5 connector) but I like having a break out box between the TNC and the radio. If done properly you can plug the mic into the break out box and talk voice too.