Locking controls to a specific heading (but not pitch!)

[Krux']

I'm playing in RSS with Realism Overhaul, so avoiding inclination correction burns is ideal wherever possible. I've figured out how to find the ascent heading to match a target inclination, but it's difficult to follow that EXACT heading through the whole ascent manually. I'd like to get perfect inclinations, especially because I'm about to set up my Remotetech network for Earth.

I have Mechjeb, FAR, KOS, etc. (and will get whatever else is necessary, but can't figure out how to use any of these to force my ship to follow a heading, without ALSO controlling the pitch of my ship (which I'd like myself and gravity to do on our own). I think I'm missing a really basic setting somewhere in one of these autopilots, but I've looked everywhere and done a lot of searches without any success, any help is appreciated.

[Snark]

Is there some reason why you can't get it started with an autopilot that sets heading in addition to pitch, then turn off the autopilot and just set SAS "hold prograde"? That ought to hold pretty much the same heading it started with, and pitch down with a gravity turn in the usual fashion, yes?

[Krux']

Yeah, I could do that, but it would inevitably lose the heading no matter how careful I am with the controls. I can get close on my own (<.2 degrees of inclination), but I'm looking to get Mechjeb close, while still manually controlling the ascent trajectory (because I've found I'm better at that part than Mechjeb in RSS).

If it's not possible I wouldn't be surprised, I just wanted to know if I had overlooked a setting or something.