You already have one... it works great in KR! I have also seen several players with ones that work in 2D - and they are not using uo-assist - but I avoid 2D like the plague so could not begin to tell you how to set one up... hopefully someone else can explain for us.
Well, KR seems to be out of the question for me, and most others. The one we have now it gives you a message that you must wait to perform this action for like 5 seconds before you can Re-equip it. (this is after the delay after you get disarmed too) It isn't like the one for Equip last weapon, no delay. The one for Rearm/disarm Left hand works some what but with a delay and in PvP you can't really afford delays.
It shouldn't be that big of a deal to make this macro for the devs. IF they don't make it I guess I will just have to adapt, if they do make it then this change won't be that bad.