Feel free to disagree, but this isn't really a matter of debate-- Khaled's policy on avoiding backward incompatible changes has been very clear. You're handwaving on the advanced warning thing, and I'm afraid you don't really comprehend the connotations of making backward incompatible changes in mIRC. There are hundreds of undermaintained (or completely unmaintained) dlls and scripts in mIRC that rely on mIRC to work as it was originally designed (+/- legitimate bugs). I personally have authored at least 4-5 dlls that rely specifically on WM_MCOMMAND and have not been updated in >3 years. I don't plan on updating them, because they are stable. Users should not have to make the choice between using the dlls they want and upgrading their copy of mIRC. Unfortunately the reality of small throwaway IRC scripts lends to the creation of a lot of unmaintained code. This is not a problem with mIRC as a community, it just means the old working scripts are stable and don't need to be touched. Khaled understands this very well, and we don't need to mess with this balance.
As far as making another constant goes, this is a little bogus-- if you're making changes in your own code, you can just as easily make the change to prefix your command with "/". If the name is confusing to you, nothing is stopping you from #defining it as WM_ENTERTEXT yourself and creating a helper script for your "proper" WM_COMMAND functionality. Most SendMessage code I've seen is wrapped up with this helper functionality already, anyway.
