I am working on something similar: fieldentity. I was planning on using that for bjaspan's combofield (#939834: Fieldentity & subform). I already have some code for that one too. To get this working we really need something like your subform_element module. I am currently working on that, but was hoping to hijack the subform module (I like that name better) for that (#939832: D7). I think I can get ajax working for subforms too. What do you think?