Can words belonging to subordinate clauses such as local clauses be placed within the independent clause? I recently saw an instance of a word belonging in a clause introduced by a relative adverb but was actually placed before the relative adverb.
Yes, it’s called “prolepsis.” The index of all the larger grammars can point you to longer discussions about the likeliest patterns and examples.
Smyth says prolepsis is used to give either the subject or object of the dependent clause greater emphasis. Is it ever used merely for metrical reasons, or is it ever used for words other than the subject and object of the dependent clause?
Unfortunately most of my reading is in verse, so I don’t have much prose to compare to. I can pull examples of prolepsis from memory, all in verse, but I think I’d have noticed it a lot more if it was a regular metrical dodge.
or is it ever used for words other than the subject and object of the dependent clause?
The only other word I’ve seen unmoored from its expected clause is ἄν.
I can’t think of any examples right now but I’d say that
a) in general it is either the subject or the object (or the predicate obviously)
b) until I’ve read and memorised everything written in ancient Greek I won’t say that a rule is absolute since they sure loved breaking some 
The tricky part is when something belongs to both the main and the subordinate sentence. Are we talking about prolepsis then? Since this is rather impossible to answer, I think we call it prolepsis only if the function of this word is far more important in the subordinate clause’s context.
Does this make any sense I wonder? That’s a tricky one too
In Latin poetry, I have seen where “ut” is not at the beginning of the purpose clause it introduces. I was just curious if Greek poets ever placed words which properly belonged in a subordinate clause into the independent clause merely to maintain the rhythm or meter of the poem as I have seen in Latin.