It's not the equipment manufacturer that applies for the STC -it's an EASA Part 21J DOA
Are you sure this is the whole answer, wigglyamp? It's very much not what I have been told by U.S. manufacturers I have been in touch with regarding them applying for EASA approvals. OK, the application may have to be routed through an EASA 21J outfit, but it appears that it has to originate at the manufacturer - who has to have the incentive to bother in the first place.
I would like to know what earth-shattering safety data has EASA discovered which the FAA has missed - given that the FAA "runs" some 90% of the world's GA....
The EASA type-specific approach is meaningless anyway. Getting an STC for say a C172 has no more meaning than getting an STC for a wide range of spamcans, because one C172 can be hugely differently equipped from another C172. So certifying a glass panel for a "C172" is just a meaningless constraint.