I actually don't think I'll write an entry autopilot any time soon. I simply don't have the time for the required test flights.
It's not technically hard - the Aerojet DAP can hold attitude precisely and crisply responds to rolls - so it's just a question of feeding it the desired roll angle as a guidance input. Using an additional PID layer, it can probably even adjust bank angle to hold an hdot target.
The issue are all the parameters of the thing, making it crisp but not overcorrecting, compensate for the lags in the system and prevent integrator windup and oscillations, setting good hdot targets based on how far we are from the nominal trajectory, make decisions how quickly we need to compensate for what amount of deviation when - all the stuff which is fairly easy to do 'by eye'.
Someone just has to create a set of such high-level decision parameters, then fly an entry, adjust and iterate the scheme,... and I simply don't have the time (nor the inclination to spend it for something that I don't really find interesting in the first place).
I'll be happy to help if someone else wants to try - it sure is a fun project, and you're guaranteed to learn a lot about the entry process itself and nested PID controllers in general. It just takes patience and persistence.