Don't fear,its just a boring and time-consuming but task if you're determined. To get the full xref tree,don't trust
IDA's 'XREFs TO' command. Instead copy the IDB in some text editor(a huge file). Start from Task_MovieRecord.
Now find the occurrences of this task just by simple text search,identify those functions and repeat this procedure
for those functions (thus do it recursively) until everything converges to task_Startup or some 'important' function
before that which has been already hooked via boot.c.
Now add some more functions which emerge from task_MovieRecord,these are responsible for bitrate changing.
The real trouble is that S5IS has more functions related to movie mode than the A-series and that will make the job
more time consuming. But in theory,it can be done by the above procedure.
P.S. Now I think you already knew all that!