Line-of-sight will dictate who to approach. No point approaching the owner of a building that you can’t see.
Apart from that, I also don’t think there is any point approaching anything remotely government-related, or telco-related — they’ll just screw you over in one way or the other.
Back in the 1990s we ran a wireless link from an outer suburb to the central business district to improve our connectivity. We ended up approaching the owner of an ‘old’ multi-storey office building that was not located on the prime shopping/business streets. That building always seemed to have vacancies, and had no lift. It was, however, only about 100m down the street from the exchange. We ended up not only getting a dish on the roof, but we got access/use of the top floor of the building in the same bargain. It even had a shower.
The owner was simply thankful that he could generate a little bit more income from an otherwise ‘underperforming’ asset, without having the usual dramas associated with a regular tenant (e.g. modifications, parking and foot/customer traffic).
My advice would be to identify a sub-prime building with clear line-of-sight, which is either obviously under-utilised or otherwise could be reasonably expected to be underperforming. Basically, pick someone to whom a relatively small amount of additional income would actually mean something and be appreciated.
You should not piggy-back off their Internet connection, or them off yours, for legal reasons. Commission your own Internet connection within the building which you have 100% control and accountability over.
Negotiate a — completely off-the-books — compensation deal them. Ideally you would either pay for this every 6/12 months in advance with physical cash, or set up an automated monthly bank transfer into an account of their choosing. Don’t ask for paperwork. Don’t ask for receipts. Don’t make it hard.
What they want is a 100% hassle-free passive income stream. Give it to them.
Around these parts a residential Internet connection (now) costs ~AU$100 a month. I would consider offering the owner somewhere in the $20–40/month if I were in your position. (Realise that you will be adding to their electricity bill as well.)