Go to
http://www.audax-club-parisien.com/EN/From the dropdown menu, click Organisations then BRM France or BRM Worldwide to find every BRM in the world.
I think the last qualifying 600s worldwide are the same weekend as in the UK, about the 22nd of June.
There are some folk who've ridden brevets with a broken foot/ ankle on a flat pedal, provided their cast allowed enough knee bend. Doing a 600 in that manner would be impressive. I think Barry Parslow did that during a long brevet (Trafalgar-Trafalgar?) back in the day and I did plenty of sub-100km rides like that. The Fenland Friends would be flat enough to allow that, particularly on a tandem, assuming mostly benign conditions.
Famously, last century one USAian rode his qualifying 200 on a fixed wheel with his left crank removed. His left leg was in a full length cast and propped on a downtube support. It was a flat route that he rode but even so, that shows some determination.
A tip from someone who had a lower leg cast on for months after leaving hospital. A fibreglass cast with goretex liner (rather than cotton wool) allows sweat, rain and bathwater to dry out, though it takes a few hours. The goretex lining is expensive but worth it. When they cut the cast off, the goretex liner doesn't insulate the skin from the cutting tool like cotton wool does (stings and can leave a scar).
Talk to a good sports physio, preferably somebody who deals with elite athletes.
