$nick($chan,$me).idle being less than 3600 is close to what you want, though I'm pretty sure it gets reset to zero from nick changes and possibly mode changes.
To have it only trap your messages, you'd need to have an ON INPUT event that would need to check if $chan == #channelname, and ignore text beginning with $comchar / unless it's /say /me //say //me. You could then add something to a hashtable with a shelf life of 3600 seconds, then later you could just check if the item exists.
hadd -m saying $chan
You'd probably want to do the same thing as you join the channel, and clear the item during ON DISCONNECT