Q: I have a 30-year-old pink dogwood that has developed a fair-sized rotting cavity at its base. Is there anything I can use to patch/fill this cavity so it doesn't get any larger?
A: I'm more concerned about what's causing the rot and whether it's continuing to spread. Some people plug holes with concrete, but if the rot is active, that'll just trap the rotting organisms and not stop more damage.
The best solution is to catch the problem early and carve out the diseased area so the tree can heal a wound or canker when it's small. Gaping wounds are much tougher to stop and heal. If a hole is big enough, that can threaten the stability of the tree and lead to a blow-over.
If this is a valuable enough tree that'd you really like to save, consider hiring an arborist (a.k.a. "tree surgeon") to evaluate the damage. An arborist might be able to take corrective action or tell you if the tree is a goner.
To get a clue about the wound status, check to see if the opening has dried and calloused over (a good sign) or whether it's still damp, dark and seeping.
For most wounds, arborists generally don't recommend painting or tarring or patching - again because you don't want to merely trap decay behind the patch.