Hopefully, I will get the wind vane a fair bit in front of the sail to make it undisturbed.
I've never seen such a thing on a sailboat, which makes me think that there's a good reason everyone puts their wind vane at the masthead. Your method is going to see slower wind speeds due to friction on the surface and will be subject to interference from spray.
On the other hand, it is true that traditional autohelms had a vane near deck level, but I suspect that that was a necessary compromise rather than a desired feature.
Edit: typo.