IMHO it takes more time to make a mIRC dll from a complete ircd than to script an ircd in mIRC script. And I still don't see the point in it. If you want to test on an ircd, download that ircd (if it's free anyways) installt it and connect to localhost. This way you can test everything without having to mess with dlls or bulke mIRC scripts.
I don't get the whole 'other OS' argument. mIRC is a Windows program, it doesn't run on a mac, it's dll's don't run on a mac. But some ircd's run on linux or if they are open source you can even compile them for any platform (with enough time and knowledge). This is more of a reason against this dll stuff
