Hi Peter,
Unfortunately, as far as I know, the PlainTalk/Apple TTS components are are all closed-source, and compiled only for use on x86(-64), and PowerPC.
With that in mind, you might be interested in the project at
https://github.com/darwin-on-arm, and the (abandoned?) blog at
http://crna.cc/, which used to refer to a project called "Magenta". The folks involved have done some of the heavy-lifting, and reverse-engineering necessary to get the Open Source version of Darwin running on various ARM SoC-based devices - but since booting, and device I/O isn't standardised fully on ARM, you'd still need to create a baseport for the BroadCom SoC used in the Raspberry Pi.
I hope that helps,
Tyson.