I know this is an old topic but I thought I'd update it anyway. I did find a way to backup standby databases using the "backup" command. It works in SQL 2008 R2 and should work in any version that supports standby. I found it here
The one requirement is that the source DB that you're backing up must
be in standby read-only mode.
Basically you create a dummy database with the same # and types of files as the DB to be backed up, offline both databases, copy the files from the source DB over top of the dummy DB, and then online both databases. Your dummy DB is now a copy of the source DB at the point in time when it was offlined. Backup the dummy DB and you have a backup of the source DB.
I would suggest also backing up the *.tuf file associated with the source DB. You'll need it if you ever plan to do a restore and roll forward the transaction log backups.
When I do this, I leave the LSCopy job enabled but disable the LSRestore job so it doesn't screw anything up.