Large Transaction Log Backups in Service Broker-enabled system
You could use a different database for your SSB load. It seems like your SSB data is transitory in nature. That would allow you to either switch to SIMPLE recovery model for that DB or throw away the log backups in case you need FULL for some other reason like mirroring.

I don't think you'll be able to reduce log usage for SSB operations. I'm interested to see other answers but usually log usage is not tunable for OLTP-style transactions. They are always fully logged. Minimal logging only applies to special cases.