For Grim; please, please don't take what I'm about to say wrong - but it's something you should maybe ponder on.
Presuming you are a caver already, is there really any need to start inventing such "contrived adventure"? Caving is an utterly fascinating activity in its own right. Rather than doing what you're thinking of, maybe you should take up a more specialised interest within caving itself, if standard caving trips aren't absorbing enough? There's so much to do. You could start by looking at the BCRA's Special Interest Groups for inspiration. Or take up aven climbing, if you want some real spice. Or any one of hundreds of other caving specialisms. Dare I even suggest taking an interest in cave conservation? (It might help put ideas of mountain biking down caves into better perspective.)
Incidentally I know at least one person who has mountain biked in GG Main Chamber as early as the 1980s; it's not a new idea.
My advice is - get deeper into caving itself. You'll get so much more from it.