I see absolutely NO reason to BS them, give them a phony address from a different street. I would not have considered that and still wouldn't, surprised that was offered as an option.
I tell them somthing like, "Subject has no address on structure, mailbox or curb. Address was verified through county maps and neighboring properties"...I have ran into this countless times with many providers and have never had a problem with it. This is quicker and it is honest to boot.
If you use a bogus mailbox, it could call the other reports into question, where by the other agent(s) reported no visible address. So then QC could go into action trying to figure out the reason for the discrepancy. And that opens up a can of worms for you.