I was trying to do exactly this with scripting, however I found adjusting the necessary parameters (the anchor position), caused BTT to crash, so while in theory it is possible with simple scripting, in practice it seems to be bugged currently.
See Updating BTTAnchorRelation doesn't work or crashes BTT for more details.
I would love to have a way to arbitrarily adjust the anchor position. My use case is that I often have windows positioned on top of each other in two side-by-side stacks, each taking half of the screen. I'd like to display a small visual indicator of the windows in the stack, to the left of the right stack when it is focused, and vice-versa when the other stack is focused. It's inspired by GitHub - AdamWagner/stackline: Visualize yabai window stacks on macOS. Works with yabai & hammerspoon.