The keys are the "key". When real plaster is applied over wood lath, the base coat oozes through the gaps in the lath and sags or expands a bit forming "keys" that, when dry, hold the plaster layer to the lath.
This is a strong structure unless a substantial amount of the keys have been broken (from temperature or humidity swings, structural shifting, physical damage). When this happens in a specific area, that section of plaster is basically unsupported and is prone to cracking and buckling inward.
If the plaster is generally sound, you should have no problem. While drywall is more flexible and less prone to pulling away from the framing, sound plaster should be strong enough to hold the weight of a thin veneer.
If, however, significant sections of the plaster are bulging outward, or if there is a large amount of the surface coat pulling away from the base coat (a sign that the plaster is in poor condition overall), this is not a good basis for a top coat beyond paint or paper. In that case, you may need a more significant redo.
If the weak areas are just a few and not too large, there are specialized plaster washers that are used to screw the weakened section to the lath to prevent further damage. That may be sufficient if the areas are few and far between.
An engine hoist with large wheels. If you can't find one pre-made "offroad engine hoist" will provide many design/modification ideas.
An engine hoist with small wheels and planks or plywood laid on the ground to make a hard surface to run on.
Same options as above but an A-frame hoist.
A 3-point hitch crane on a tractor.
A small excavator (rental, one day should do if you are prepared.)
A hitch-mounted crane on a pickup truck.
Best Answer
Don't seal the brick. It will only prevent adhesive from bonding correctly. If the brick is loose remove it, vacuum the dust and re-install the brick.
If the mortar is loose scrape it out and re-point the joint with mortar or concrete caulking. Allow the repair to cure.
Brush or vacuum the brick face aand joints to remove any dust and loose debris. Same for the veneer; clean the back surface.
Get a cartridge of construction adhesive and load it in a caulk gun. I've had good results with PL400, but any brand name will do. Apply gobs of adhesive about the size of a quarter to the bricks face.
Press the veneer back into place trying to make the broken seam as tight as possible. Hold the repair in place with strips of duct tape.
When adhesive has set any gaps along the repaired edge can be filled with concrete caulk to keep water out.