Indeed. I would put it like this: the fundamental issue here is that /signal, including /signal -n, does not pass on event-related identifier values to the signal handler. As another example, if you use "/signal -n" from "on TEXT", you'll find that in the corresponding "on SIGNAL" code, $chan returns nothing and $nick returns your own nick. For someone who does not realize this, I think that documenting that $mouse also works outside events is not going to be of much help, even though it is a good idea anyway. This is simply an issue with rather misleading symptoms. As Wims mentions, the solution is to pass the desired event values as parameters to /signal.
