Yeah, it seems like there are two potential ways to do this that could work - both basically putting the existing "Defend" behavior onto a mobile point. That point could either be explicitly a ship, or it could be simply the center-of-mass of a bunch of ships (though that might be harder to to convey UI-wise). Retooling "heavy escort" to be this could well be the easiest (and most clear) way to go, though it has the downside of breaking the group up if that ship goes down or retreats.
Without modifications they are totally going to block the firing lines of the 'rally ship' though, which isn't ideal.
This should hopefully address that aspect of it.
Regardless, this probably isn't in the cards for the .1 release - seems like too much playtesting / messing around with it to see what's actually good, and how useful this is (thoughts very much welcome!) would be required - but I'll definitely keep this in mind!