It would be nice if the forum software could automatically link to changesets and tickets, like Trac already does. So if I simply write #837 the forum makes #837 out of it. And the same for rXXX.
I do not know if there is already a suitable plugin for MyBB, but it should be also easy to make one for somebody who knows how MyBB works. It would simplify writing posts very much.

i think, this only can work as a bbcode. I will take a look, if some Plugin exist.

But it works with http://-links also - without bbcode. I've already seen other forums with similar functionality, a simple Regex-search and replace should do it.