I remember from some own multicasting implementation I did with Bonjour as an inspiration that multicasting should have no problems with subnets actually. Routers may just decrease the TTL (Time To Live) value, so multi-casting packets may die before reaching other subnets if the packets are send with a low TTL value and the routers dec a not-so-small value (and it probably makes sense for APs to reduce it more than standard routers). So maybe a custom option to set the TTL values could be an easy thing to try?
edit: I checked Firefly, iTunes and the Roku – they all use a TTL of 255, so the above is not the reason.