Apple says specifically don't do it.
See BTMM and iCloud instructions.
http://support.apple.com/kb/ht3486
And I quote.
Can Time Machine remotely back up or restore data from AirPort Time Capsule or its attached hard drive via Back to My Mac?
Backing up or restoring Time Machine backups remotely (that is, via Back to My Mac and the Internet instead of your local network) is not recommended. If you want to back up or restore with Time Machine and AirPort Time Capsule, you should do so while directly connected to your AirPort Time Capsule's local network.
I think Apple have good reason to recommend you don't do it.. eg.. you can easily corrupt the entire backup. So instead of having an up to date backup you will have none.
Use a 2.5" portable drive when travelling. When you get home.. backup to the TC as usual.