Indeed, Cremation does give a value in between burying and the other methods you mentioned. If a regular bone gives X experience for burying, you will receive 2.5X prayer xp and 2X fm xp per bone cremated.
I've mentioned and in fact wrote up something for it to be put into the regular page on Cremation before, but that idea didn't seem to catch on since it was so simple and apparently not worth adding.
Anyway, I'm looking into the calculator right now and it seems that adding it should not be a difficult task . Are there any special types of bones that cannot be cremated that we should know about? As far as I know things like regular, big, dragon bones can be cremated, and ashes cannot, i.e. 'everything Prayer with bones in the name' can be burned but not the other stuff.