I don't have anything proven to work to report. I have done hundreds of brake jobs on GM trucks and other GM products and plenty on my EVO but I have never encountered a directional noise?
Is it just a nuisance squeal that you can live with, or does it sound like something is in contact?
I use grease or the sticky anti-squeal compounds too, but I have never had 100% satisfaction from them.
You may try putting small dabs of grease between the caliper clips and the caliper mount, and then another between the tab on the pads and the clips. The noise can be coming from any of those spots, but they are hard to grease being so close to the rotor, so I usually use a q-tip or toothpick to apply grease there.
The one thing I will pass on that sounds weird but 100% removes pulsing from GM disc brakes was a tip I read in the GM service manual on my Tahoe. When tightening the caliper bolts, do not allow either wrench to make contact with the caliper or caliper mount. Sounds weird I know, but I can say with certainty that if your wrenches make contact you will feel a faint pulsing in the pedal especially under normal residential neighborhood type braking situations, and if you are careful not to allow your wrenches to make contact you will not have the pulsing. I'm sure this has nothing to do with your squeal in reverse, but since vibrations create noise, you never know...