IO540, I think wigglyamp has pointed you in the right direction for where to look in the AMC / GM.
On your point about the FAA, a PMA is in some ways similar to a minor mod (in a previous job, we used minor mod's, done internally with no reference to EASA, to certify after-market foams and covers for airlines, which is sort of how PMA is used in the US). A TSO is a parts approval independent of an aeroplane (or it is for seats anyway). So, if you are fitting parts with PMA or TSO then someone somewhere has already done a lot of approval work with the FAA and is recovering the associated costs in the piece price. No difference from EASA that I can see.
EASA has an exactly parallel system of ETSOs. To apply for one of those, you need Alternative DOA procedures approved by EASA. I have just done that where I work now and it is not onerous or expensive. Bizarrely, a DOA does not allow applications for ETSO so my organisation now has both DOA (for minor mod's) and ADOA (for ETSO). We don't do STCs.
One does not need a minor mod' for every individual aircraft, if Bloggs DOA has written a minor mod' to fit a Scruggs ABC transponder to all TinyCo 123 aeroplanes then you can use that mod' more than once. The difficulty is identifying who, if anyone, has already written a mod' to fit the kit you want to the aeroplane you want. The only way I know to do that is identitfy all the DOAs with relevant privileges and ask them if they have what you want which is a bit of a faff!