appositive relative clause or adjunct of reason/cause?

Solution 1:

Here is an answer to your question about this:

"They then took the matter to a three-person jury of appeal [that was] specially convened to hear the case."

[I am changing protest to case, but that does not affect the grammar]. The comma would make "that was" irrelevant.

my answer on ELL

The two criteria for appositives are: they must be right after the noun with which they are identified, and they must have the same grammatical structure.

There are times that an appositive can be out of place (as the one in the link) but in your case, "specially convened to hear the case" describes the three-person jury of appeal, it is not a "renaming"of it.

It is a non-restrictive postmodifier phrase describing the jury of appeal.

I guess some might consider it a relative clause if one wishes to see it as "that was specially convened to hear the case."

post modifier