The PRR E2b prototypes were equipped to operate in MU with the older P5 fleet. That is probably why they had 32-volt rather than 74-volt auxiliary electrical systems. There is credible confirmation in a book, including a picture of the E2b MU sockets with appropriate labels, but I cannot find my copy right now.
The GG1 MU capability I do not know about.
I have read - on the forums I think - that some of the E44 fleet were retrofitted to work in MU with trailing diesels, but I have not been able to find any authoritative confirmation that this was so, and if it was, the details of how it was done. One of the potential issues is that with a manually accelerated high-power electric locomotive, the engineer may well still be notching up at 30 mile/h and above, whereas the trailing diesels are probably advantageously gotten into notch 8 by about 15 mile/h. Not a deal breaker, but it would appear to require careful mapping from the electric to the many fewer diesel notches. (A similar issue would have arisen with the UP GTELs when they were retrofitted to MU with trailing diesels, but the details of that system remain stubbornly hidden.)
For electric power units to be controlled by diesels, I think that there would need to be at least partial automatic acceleration. In this regard, the New Haven FL9, in electric mode, was controlled in 8 steps by the diesel throttle, but between some of the diesel notches there were several electric notches under automatic acceleration control.
The only case I know of where electric locomotives could be controlled by leading diesel locomotives was in the UK. The BR 33/1 class diesels could control trailing 73/1 class electrodiesels, including selecting whether the latter operated in electric or diesel mode. This was because both classes were equipped with 4-notch EMU-type control systems as well as their respective own underlying control systems (two, one each for diesel and electric in the 73/1 case). Both could also MU with many of the BR Southern Region (SR) EMU consists. The BRSR first generation DEMUs were nearly brought into this group, and BR’s class 210 second-generation DEMU prototype was equipped to work with EMUs of its own generation.
There may well be other examples of diesels controlling electrics in Europe. With the advent of electronics, heterogeneous MU would have become a lot easier.
Ideally I suppose, one would want a control system that maximized electric locomotive output in a given situation, and used the diesels as supplementary tractive effort and power only to the extent needed.
Somewhere I have seen it said that the VGN E33Cs could MU with the EL2Bs. That seems unlikely, particularly in respect of electric braking. So that’s very much a maybe that definitely needs authentication.