In terms of "social engineering techniques", I see nothing wrong with asking which apartment he lives in, given your goal in this most unfortunate situation.
Alternatively, covertly surviel the mailboxes. Then you get the address, at the very least. You could then approach building security as mentioned before and ask who lives there as you need to send them a letter, or something similar.
I'm still struggling with SPCA's attitude. I mean, is it really up to you to prove all this? I'd have thought they would have had a standard investigative/complaint procedure - I mean you live in a
condo complex, right - these "people" could live anywhere.
I did have one other thought. Is the playground area covered by video surveillance by your complex? I'm guessing if you have the date/time of the incident then you could tell SPCA and they could request the coverage. Maybe even building management or security themselves would be able to help?
Just a thought...Mr. P