There may be a case here with DOT. The ACAA is 14 CFR part 382, the full text of which may be found here:
http://airconsumer.dot.gov/rules/382short.pdf.
The relevant portion is § 382.38 (4) which reads:
"For a person with a fused or immobilized leg, the carrier shall provide a bulkhead seat or other seat that provides greater legroom than other seats, on the side of an aisle that better accommodates the individual’s disability."
I should think that your leg brace would qualify as immobilization.