Hi Neels,
First of all much appreciate your detailed answer.
I will monitor the git channel so I can do some tests when a merge happens in this regard.
Until then, I found some out of tree codec support for Asterisk:
https://github.com/traud/asterisk-amr https://github.com/traud/asterisk-gsm-efr and even https://github.com/traud/asterisk-evs
The last one is not necessarily interesting for 2G, the first two I compiled in but never had enough time to test it as there are enough issues with the Nokia BTS support I can deal with. So I can only tell that AMR and EFR codecs are compiling and loading fine on the latest Asterisk. FYI the AMR module can do AMR and AMR-WB and transcoding, the EFR module can do passthrough and transcoding as well. Maybe this is interesting for others as well.
Regards, Csaba