This is a cheesy way to do it, I'm generally against doing things the cheesy way, but these signets are really annoying and take far too long the "right" way.
Have your group by the zone in, get the expedition, run in, see if the signet mob is up or if it's a PH. A necro is ideal for this (we'll get into it later). If it's a PH, leave the expedition and get a new one until it's the sig mob. Also, make sure it's the right instance... it's harder to get the right version of the zone than to get one where the Sig mob is up.
Once you get the sig mob, get as many people to him as possible... in catacombs, stand in the cubby behind the sig mob, in sewers, there's a hall behind him with a murk you need to kill first, but it's pretty clear. If there are see invis mobs (which there definitely will be in Sewers, and possibly in Catacombs), drag the bodies to the necro and get them rezzed. You, as the bard, may need to mez the mob while rezzes are happening, or just kill it and rez up afterwards.
At this point you can either sit there and hope for a new sig mob every 20 minutes or drop the expedition and repeat for everyone who needs it. We managed to get 3 catacomb signets and 2 sewers signets in a little over an hour like this.
--Trebla