Interesting idea.
Afaik Minor Roofs are only attached by press fitting - being aluminium. The Traveller roofs are held on with pin nails straight into the wood!!!

So, what you're sugesting sounds perfectly OK in terms of 'good british engineering'. I imagine Morris/Austin would do the same if they'd made the mini these days.

Besides, if you have a cage you wont need the negligable strength the roof 'skin' offers. Just make sure its all smoothed so finnicky MOT men cant moan.