TSO-C199 was still in draft when the proposal surfaced. I think you can take it for granted that it will be a permitted source since it is such for the currently existing FAA ADS-B network - excluding it would make little sense and technically this would be even worse since the space based system would feed into the ground based network and you would now have differing equipment levels sending out the same SDA and SIL and you would have some that may be legal and others that are not - it would break the whole SDA/SIL reason for being...
TSO-C199 is actually a very good standard and builds a wide bridge to make ADS-B affordable, yet maintain safety and accuracy without compromise.
CAA usually takes over whatever FAA says - and adds the standard to its own (calling it TSO-ZA). But who knows these days ?
BTW, if you have a Trig transponder and would like to continue using its own head you cannot use the iEFIS as qualified GPS source - that is only possible if you have the iEFIS control the transponder (i.e. no Trig head). There is however a way out: Our SP-12 GPS has the FAA certified GPS module and this is compatible with Trig's TN72 GPS module (Actually just about all manufacturers now use the same GPS module). You can use the SP-12 in place of the TN72 if you want to save a bit. The SP-12 is qualified to SIL/SDA of 2/3. That's as good as you would ever need. The iEFIS built in GPS is qualified according to the provisions of TSO-C199 to 1/1. This is good enough for the ground based ADS-B system in the U.S. and it will accept your transponders transmissions and reply with traffic, notams, metars, Nexrad Wx and all the other ADS-B uplink goodies. It is 2020 compliant as ADS-B position source.