Is there any way you could attach it to the outside of the tower, maybe on the front legs or the side support? I know I have a different tower, but this installation has worked really well for me. I got the tower mounts for the bimini for about $20 at diywake.com
Another thought after looking at your picture some more, I bet if you move that bimini support base forward about 18", you could simply let the bimini rest in front of the tower, still keeping everything kosher for boarding while the tower is up. That's probably your cheapest option. Might have to add/reposition the hooks for the nylon straps too. Good luck!