I have been scouring Jelly Bean to see if I could find the Offline Dictation feature but I have not had a lot of luck. As you know, the source is huge and cumbersome. I found the /external/srec folder but the documentation in there seems to be from Nuance when Android was first launched back in 2007. Is this only available as a closed source Google Application (Gms)?
Unfortunately, I don’t think it is part of AOSP.
There is an IME called VoiceInputMethodService which is likely where the offline dictation comes from. This IME exists in ICS under VoiceSearch.apk, which in JB was renamed to QuickSearch.apk
Comparing the classes from ICS and JB for these two projects you can see a lot of new classes related to voice recognition under the Jelly Bean version. New classes and packages with names like “Majel”, “Recognizer”, and “decoder”.
I don’t have time to go through it all tonight, and I still need to confirm that VoiceInputMethodService IS the IME used for offline decoding, but hopefully this points you in the right direction.