...if there's a way to record 'b' as yielding 'both' and 'bottom' in different circumstances.
Well yeah, and 'block' too.
Of course it'd be great to have context based completions.
My problem is there isn't a concrete commitment to one particular method as far as I can tell.
It seems to me, being on the periphery of things, that sublime-completions is where further development is going to take place in regards to specific context completions. And if there are limitations to using sublime-completions file for describing tab targets, I'd love for them to outlined and addressed.
I don't mind scripting a quick and dirty converter for either sublime-keymaps or sublime-completions, but I'd like to do it for at least the one that promises to provide the most features (in case the snippets need refinement as you've mentioned).
On the other hand, if the only way to improve snippet behavior will be through a plugin, then the method of defining the snippet is irrelevant, so long as Sublime is able to find it. If that also means that a sublime-keymaps file is the best method of accessing the tab target information for fancy context based snippet plugin, I guess that's fine too.
But this architecture needs to be figured out and promoted.
Anyway, good to hear your thoughts on this.