This will probably be beyond my knowledge. Looking in F1 help, I don't see any EVENT that would respond to clicking on a tooltip. Also, there are other tooltips that can happen, such as having the ON DISCONNECT and ON CONNECT be scripted so that they produce a tooltip when those happen.
The only solution I can think of is something that's not what you're asking for, which is to assume that you want to see the channel where your nick was used when you restored mirc from the tray.
It would work by adding code to my previous $highlightevent alias
If you're highlighted, the alias would need to monitor $appstate to see if mirc is currently in the tray. If it is, it then saves a setting containing the name of the channel that highlighted you AND THE $NETWORK, otherwise it would make sure to clear that setting.
It would need to monitor ON APPACTIVE, so when it detects that the active state changes from $false to $true it would check to see if that earlier setting contains the channel/network of the LAST time you were highlighted. If so, it would then find the correct network to activate the channel.
Note that this is different than what you wanted, because it would always activate a channel if you'd been highlighted there while minimized, even if that happened 5 hours earlier. I guess this could be modified to safe the timestamp for when you were highlighted, and only do something if it's recent enough.