Awesome info there, thank you for sharing your deliberations. Agreed, 5HP24 will need at least a later type EGS module.

And certainly your project will need an 5HP30 in the long run. It will be interesting to hear your thoughts once you progress, as 5HP30 comes from many later cars, so it is possibly you will get one from an E38/E39 and be able to make some checks? Or even
an X5 (if that one really fits), it seems 5HP30 from all these can work with e34 and e39 EGS. Though this needs to be checked physically.

My suspicion is that the e34 (ADS interface, non CANBUS) ABS, EGS and ECM collect the required drivetrain data, eg wheel and throttle speed information as this is used to implement ATSC (on a 540 and other ATSC
cars), but not throttle (E34 built with ATSC+T may implement it by closing the injector valve timing), perhaps M5 with fly by wire does. My hope is that the information these collect is enough for late GS 9.20/9.22 equipped ECU to work the steptronic features as per e39 540i.

If so, to retrofit ZF's 5AS steptronic transmissions, it may only be necessary to implement an e39 fly by wire throttle setup and/or CAN throttle position sensor, and of course it would at least require CANBUS twisted pair between the 9.22 EGS, e39 throttle sensor, kickdown switch. I do not think this is hard to do. The ABS sensor is at one end of the CANBUS and is used to terminate one end of the network. Without an e39 ABS sensor, it will just require fitting a 120 ohm resistor between the yellow/black and yellow/brown Twisted Pair. It may be worth studying the OBDII E31 840ci ETM/TIS for more clues- or alone early 1995 E38 and <10/1996 E39. I do not know which module contains the termination resistor for the other end, it is likely to be in the EGS, ECM or the cluster. My guess is the cluster, so to setup a CANBUS between an ad-hoc bunch of e31/8/9 modules could take two resistors, one at each end. I totally agree, the greatest candidate for a 5AS auto transmission upgrade is the 535i/735i, as once M30B35 is done, B34, B33, B32,B28, as well as B20
powered cars, all would be in scope for anyone to do. I think however that later ECU may be a requirement to implement an early steptronic EGS, so M50/M52 powered e34s would be the ones to do after M60.

It would however be truly great to open up 5HP24 upgrades as a DIY mod for any 6 cylinder E34, E32, E31, E30 E28, E24, E21, E12, E9, E3- that's a lot of BMWs that need new gearboxes that are freed up from all those 'naughties era' models going to a premature death right now.  Standard/manual transmissions are not always the right option for people with older cars experiencing automatic trans issues, after all.

Re EWS and DWA, I am not aware of the EGS being involved in the key exchange from the transponder controller, but it may operate the gear knob lock solenoid on the shifter. On E39 this solenoid has a two pin connector (simple 12V control) which does not
appear to connect to CAN module in the shifter switch. I just looked at an early e39 shiftronic shifter assembly I have here - it uses the physical selector cable to select gears on the transmission directly, and uses two microswitches to implement the up/down side. Both, along with the main swiper switch talk to the EGS via a6 pin connector.

Not sure if this helps shed any light for you, but the closer I look, the more it appears possible!