Well, my answer is in two parts.
These days I'm thinking in terms of my own GPS, a Garmin GNS 480. This unit supplies ARINC steering "consumable" by an autopilot, which effectively gives you a proportional steering signal that could be displayed as an FD. I believe other units such as the 430/530 will also, at least in GPS mode, provide such steering instructions.
In the old days I can't really tell you exactly what the F4 FD was telling you. It may have been supplemented by the inertial nav system when flying an ILS; all I know is if you used the needles it did a pretty good job of keeping you on course and glide slope. It also functioned in certain bombing modes, notably loft (always a favorite, hurling bombs 2-3 miles away!).
Anyway back to your product - if the GPS already gives ARINC steering commands, can't that simply be converted to a visual steering indication? If you delay a turn, the bank directed would be steeper, etc., up to bank limits?
I know the TT simply follows steering commands from another source. I don't know if your AP does the same, or if it interprets it some other way - but even if it does, it ought to be possible to display external steering data irrespective of your AP.