Indeed, i think a right click context menu could be handy if done correctly, but i stick by my opinion that registering the filetype would be a messy implementation at best and as such a bad idea.
In response to a few 'arguments', opening/loading a file in every running instance/installation of a program would be silly behavior, because such an action would almost NEVER be wanted. (compare it to opening a text file in all notepad instances...)
Mirc is 'move-able', you can relocate the directory anywhere and it will function just fine, and yes including ssl since mirc simply checks system and mirc folders for the dll's on initiation. (only restriction i can see is badly coded scripts not liking spaces in the path)
