This is not a standard situation for sure. So, you're living in a building still partially under construction?
For your mailing address, you can use anything, I'm sure you understand that. As long as all the incoming mail from the visa office will get to you safely and promptly...
Residential address history from the form would be used to run background checks on you. So, in my opinion, if if there's an alternative way to give your residential address, whatever it is, then use this "known as" or "customary" address instead, not the official residential PO Box... For example, if I was to come visit you from Canada, and I needed to show your address to a taxi driver, what would you write for me? A PO Box?? I highly doubt it. It would be some sort of a street address, building and apartment number, whatever... Or, another example, if the Saudi police would stop you on the street and ask, where do you live? What information would you give them? Then use this form of the address, because the logic of giving your background info is to tell where you physically reside(d). Provide the PO Box addresses on an additional sheet of paper as an addition to the form just in case...
If there is no way to do it, then, as the last resort, put down whatever the official address of your residence is (or was), even if it's a P.O. Box and provide a good explanation in your cover letter just in case, based on the fact that it's common practice in Saudi Arabia, to prevent them from requesting more info (causing an unwanted delay...)
That's what I've always done in each of our immigration applications, if there was something out of ordinary, I've explained it very carefully in the cover letter providing all facts and documentary evidence that could help clarify it.
It's okay to provide more information, as much as you feel needed, but not okay to be incomplete.
Understand that your obligation in filling out the forms is to the best of your ability to reflect the actual situation. So, if you're living in a "shack 10 near the construction site such-and-such, Ha'il city, Saudi Arabia" then that's what you'll end up indicating... So, don't use another address (like, your employers address) just so it looks better...
It's just my opinion based on the approach I prefer of always being as truthful as possible, but it's not a firm advice on what you should do... I personally don't think that it will cause any processing delays if you give truthful information, whatever it is.