Minor mod's by an EASA part 21J Design Organisation do not go to EASA. If you don't have access to someone with a DOA then yes, the mod' goes to EASA (or NAA) for a Technical Visa.
The definitions of minor and major are well covered in the part 21 regulations and guidance material.
Organisation Approvals | Design Organisations Approvals
(it's not a rivetting read!)
There is a list of DOA holders here:
http://www.easa.eu.int/ws_prod/c/doc...o/easa_doa.pdf
You might find what you need here. If you are really lucky, you'll find one who already has approved the mod; you require.
List of EASA STCs here. I do not know of any central list of minor mod's because, I have said, EASA does not see them.
http://www.easa.europa.eu/ws_prod/c/...ber%202008.pdf
I have to say that I don't get all this EASA bashing. I have worked under EASA DOA for the best part of 5 years and find the processes entirely viable and the people entirely helpful.
Sadly, I cannot help with GA aircraft electronics, my EASA recognised skills are in large aircraft interiors.