On Thursday, May 19, 2016 at 2:50:00 AM UTC-7, wrote:
"The installer does have to obtain permission from the original STC holder."

Does anyone else see that as a possible problem?

Unlikely, the FAA document provided is very clear.

Obtaining the original STC costs money so I wouldn't be surprised if the holder might be reticent about letting people use it for free.

Not likely an issue. In practice most of these ADS-B AML STCs were funded by the ADS-B manufactures and available free of charge for many GA type installs. Those vendors care about selling ADS-B hardware to a broader market not STC paperwork.

In practice an install shop would either use an STC or setup parameters published by the ADS-B hardware vendor for a ADS-B/GPS pairing. And vendors can now provide setup instructions that don't require AML STC paperwork.

As I understand it you need an STC for installing the unit in the aircraft and another STC to connect the ADS-B unit to the selected GPS source.

No. Any single ADS-B installation STC has always covered all that. The very core of any ADS-B AML STC is that it is the installation of ADS-B hardware with a paired specific GPS source.

The ADS-B in function of the PowerFlarm has already paid for itself in my glider though and I fly in Canada where ADS-B isn't even on track to become mandatory.

Not sure why you are worried about FAA approval issues then.