Hello Thomas
as it's about the EU PAS rule, there are certain ways to cope with it.
I did use PAS to trigger throttle (Voltage 0-5V via a simple frequency-voltage converter) and brake (same frequency-voltage converter it just detects reverse and triggers a brake-ground).
With a simple adding you could use PAS as primary and a throttle as second input. It took me about 10 pieces of electronic to put it together, so no fuzz at all.
How the MP3 does that internal sounds like Alan already discribed it, but I did not test it myself (as my third MP3 is in replacement atm which should do PAS only)
Cheers
Sam