No answers, but I can share some experience. The only ways I could put a shoulder on any modern film (and by this I mean FP4, HP5 kind of stuff - basically not old style emulsions like Super XX) was to 1) drastically reduce agitation with hugely increased development times (and I mean drastically reduced agitation - like once every 3-5 mins) and 2) use a water bath. Obviously both methods come with inherent risks of uneven development. The mechanism is also physical rather than chemical i.e., starve the highlights of developer. Rotary development (with about 25% reduced times) provided identical curve shapes to those I got with my usual agitation pattern. I agree with your central thesis - if you want a shoulder, use a film with an inherent shoulder. There does not seem to be any chemical way to induce a shoulder (unless it is combined with a physical way like reduced agitation). I think it is almost easier to go with masking techniques of some kind to alter gradation.(posted 8074 days ago)With regards to adjacency effects, Richard Henry reported that adjacency effects seemed to be primarily due to bromide diffusion within the emulsion rather than on the surface. I have found pyro, Rodinal and D23 to give very differnet adjacency effects with rotary processing, which would seem consistent with Henry's findings. In other words, you don't need to run the risk of uneven development for adjacency effects.
Cheers, DJ