You know, I've just realized that the $$? isn't necessary, because you can specify it through $1 as such:

tnick $+($me,|,$1-)

so when you enter, for example, /away brb, the script will then change it to the temp nick Jethro|brb

I just don't see the necessity to go the extra mile to engage in a popup prompt, especially this is simply a self-trigger alias.