Quite often on yachts you see a wind indicator or instrument head mounted forward of the top of the mast. If you want something like a windex to indicate the tacking angles, rather than a simple burgee, I guess you could put one jutting out above the forestay on the main mast on a BR, thought it wouldn't be much use on a run or broad reach (not that you'd run if you could avoid it given the assymetric on a BR.
I'm some way from being able to try this out on my BR 17, so I'd also be interested in any shared experiences.