I have been thinking about this today and after all that thought have still not decided what I think about it all
Tunnels to me have always meant purpose built things but I know that natural tunnels are also 'allowed'
I have little or no experience with BG's, but have a fair bit of other trapping experience to call from and I personally wouldn't chance using a Fenn or Imbra in that kind of set.
It does all come down to 'what is a tunnel' and what is the meaning of 'purpose' in the HTAO doesn't it? And that, I would think would be subject to the interpretation of the prosecuting