Backup to a time capsule on different subnet by IP address?

Is it possible to set things up so that a computer on one subnet can backup to a time capsule on another subnet, addressing it by IP ?

Here's my situation. My home ethernet network has 2 subnets. One is in a public IP block of 8 IP addresses, the other is a private IP block. My time capsule lives in the public block. There is a router connecting the private and public subnets (the router's LAN port is on the private block, WAN port on the public block). My Macs on the public subnet can connect just fine (over ethernet - wireless off) to the time capsule and back up.

Is it possible to get Macs on the private subnet to see the time time capsule on the public subnet and back up to it *over ethernet* ? The Macs on the private network can ping the IP address assigned to time capsule just fine -- but they can't access the time capsule via time machine for a backup.

(Yes, I realize I could backup these computers wirelessly by connecting them to the time capsule wifi itself, but the locations of the computers in question makes this impractical. I want to do this over ethernet).

Thanks for any suggestions.