Well, I have Alpinestars motocross boots (no clue what they're called, Tech 6 or something). Only used them once or twice, didn't have my dual sport long enough. They're really stiff compared to a roadracing boot, I would have to move my entire leg up to upshift because the thing wouldn't give in the ankle. They would seem too bulky on a sportbike though.
Can you fit a brace inside your current boot? Get a larger boot to accomodate a brace? Fit a brace AROUND your current boot? Get a Brit bike that has the shifter on the other side?
I think Sidi has a newer one with an internal 'bootie' with a brace that limits excessive movement. Saw 'em at Euro Cycle Specialties here in SoCal. Not sure that they were Sidi's though, and they were more $ than the Vertebras.
If it's the right leg I wouldn't worry. Just don't use your rear brake for a while (I don't anyway). If this is only a temporary injury you shouldn't really have a problem (of course it's an excellent excuse to get yourself some quality riding boots like the Sidis mentioned, the Alpinestars Supertechs, or the Daytonas).