You can use an on text event to catch certain letters, words, phrases and put them in the clipboard that way.
on 1:TEXT:*:#MyChannel: {
if Blah isin $1- { clipboard $1 }
}
That would put the whole line into the clipboard (after clearing out whatever is in the clipboard first).
Edit: I think I misunderstood. You want something that when you drag the mouse across text on screen that copies it to the clipboard? Just hit control-insert.