Verbal nouns with auxiliaries
If we have a construction like "kuhusu kilichokuwa kikitokea" (sentence 5202 in the global voices data), it becomes thorny. UD wants us to not have auxiliaries as the heads of their clauses, instead the verb that follows or in the case of copulas, the noun that follows should be the head of the clause. However, in a construction like this, kilichokuwa is a verb with relative marking but no overt noun that this simple relative is modifying.
As such, this is a derived noun/nominal relative clause. It is functioning as a noun (the preposition kuhusu is a case dependent of it). Because it is a derived noun, it is contentful and not purely functional. Thus, kilichokuwa is the head of a noun phrase. If the verb that follows (kikitokea) was another relative, then the verb that follows would be an acl dependent of kilichokuwa. However, in this case it is not, so it kikitokea is a ccomp dependent of kilichokuwa instead.