I'm not so sure about needing permission (other than to be polite).
I haven't read Calareymud's specific licensing issues, but most of the DikuMUD license requirements are that credit remain intact, and that the code not be used for financial gain, which says nothing about redistributing or refactoring it.
Github does not have any kind of GPL requirement like some other sites do, so while you can't post it as an "offical" Calerymud release, you could probably toss it up there as a bug fix release and shoot an email to their devs asking if they mind. Just make it clear in the README that it's not official.
If the license says you can't distribute modified code, then forget I said anything.