I would suggest that -okdir should follow -ok unless there turn out to be some very user-surprising consequences.

I haven't had as much time for findutils lately as I had hoped, so if you beat me to it, I won't mind.  I'm still undecided on whether -okdir should behave the same as the (eventually) fixed -ok or keep its current semantics of redirecting stdin from /dev/null.

Should we for whatever reason decide not to change the code, -email is unavailable- suggests an example:

JY> If you would like to propose an illustrative problem for which
JY> your workaround is the most effective solution, we can probably
JY> include that in the documentation as an example.

OK, please mention something like:
-ok in contrast to -exec, has its stdin redirected for various reasons.
To workaround this, where one would normally use -ok, one should use e.g.,
-ok true \; -exec ed {} \; #to edit selected files
-ok true \; -exec mplayer {} \; #to play selected songs

Note, however, that -okdir remains a GNU extension, and that as such, it should probably continue redirecting stdin from /dev/null (since we can claim, as part of providing the extension, that it is more predictable if child processes can't interfere with our ability to ask which commands to run).