Alright - so an update from me after many months of puttering along trying to sort this out.
I've managed to get to a pretty good spot through my superannuation (AusSuper) insurance (TAL) by simply re-applying for my coverage. Basically every 2 years you can re-apply or just allow your coverage to continue. Given that I know no one is going to read back on this thread - 2 years ago i applied for extra Life and TPD insurance beyond the standard to roughly $1.5m each. It asked me about mtn biking specifically (along with other 'extreme' sports) and I ticked the box that I participate. That then meant mtb was an exclusion on my policy for everything.
Recently i was told to re-apply for the insurance which i did, which means you need to reconfirm your details including the participation in your "pastime" that you declared the first time around. However, this time, mtb is no longer an exclusion from my TPD and Life, and is only an exclusion for Income Protection but even that is only for over $6,000 (policy set to $6,500).
So i finally got a good result without having to go through any major hurdles.
I do appreciate the guidance i got from some others on here (
@teK-- ) and would have gone through a broker he suggested had this not worked out.
For everyone, particularly with mortgage and family, I personally don't think it's worth assuming you're covered because it never says you're not and it's worth taking a close look and being proactive to ensure you have it somewhere in writing that you have coverage (of course dont blow your cover if you're confident you are covered and end up with an exclusion!)