Question: there’s a 1500 foot tower in my vicinity, but not in controlled airspace. I have no contract with the owners or operators. Can I fly up along side it to the top (+400) as a 107 pilot?
Yes, as stated above, so long as you're flying under ALL part 107 rules and not entering "controlled" airspace etc. Now the fact that you might be trespassing on private land in order to take off and remain VLOS (you'll want to be close to maintain clearance from the tower for safety).
Also for others reading this, in order to utilize this "allowance" it must be a structure so something "natural" wouldn't suffice in allowing you to fly higher than 400'AGL.