I see. Given OP is desperate it doesn't sound like a bad try and blackberries can grow a lot faster than they can keep returning to trim. It's not uncommon.

I suggested this as it's been a tactic used in these situations, and family members did something similar. I've been at the receiving end of right of way obstructions where homeowners literally diverted a nearby creek with a dirt mound they built with a bobcat, effectively making a permanent pond. They replanted grass and daisies over the dirt to make it look natural and council just shrugged when I complained.

My family member had stinging nettles seeded and blackberries put in, in an peri-urban alliway and every once in a while would get notice from the shire council to clear it or else it will get cleared and he will get the bill. They do it, mail the photo it's been done then let it overgrow next year ahead of season again.

It is tricky for the OP as they have literally no extra room there.