I see two challenges if trying to use Fremantle audio stack as it.

First to, to use APE centric audio[1] model that was used in N900 you need to implement the libcmtspeech interface[2] for the new modem.

The second problem is the alsa mixers. If the used codec is not exactly the same then there is a need to emulate the necessary parts of the N900 alsa mixers and fake the rest.

Other option is just to drop the old stack and use pulseaudio as it is today, but then you loose audio policy.

Back porting Harmattan audio stack for Fremantle sounds like a viable, but very cumbersome option.

[1] In APE (application engine) centric audio the cellular call is routed via phone operating system audio stack (in N900 case trough pulseaudio).

[2] The libcmtspeech implements the interface for routing the cellular call audio to linux userspace. AFAIK the original code is opensource, but I have no idea when to find the Fremantle level sources.