pmezard:
  Won't that lose the automatic aspect of imerge merge -a (it'll just run what
you have 
  configured, which may be interactive)

I think using the builtin simplemerge instead of calling merge(1) would work
(but haven't tried).  It's somewhat tempting to suggest that the new merge
framework gaining a .interactive property, and imerge using those tools where
.interactive == False in priority order (as appropriate) would probably be the
right thing (presuming here that simplemerge would be the default case there,
where nothing else was configured).