In the locally hosted mudmode specification, you have this fragment:
Technically, you are correct, the key in an LPC mapping can be of any type, tho arrays and mappings tend to not work as you'd expect.
For I3, only strings are used as keys in a mapping, and it is safe to assume there are implementations (esp. non native ones) that will not handle non string keys.
I will update the mudmode specification to mention this.
intermud.org - Forum
http://www.intermud.org/forum/viewtopic.php?showtopic=6