So far I've checked cornerstone and versionapp, neither of which allows to do that.
I've also tried mounting a remote disk (SFTP) using Transmit 2, but that resulted in an error when I've tried to checkout the SVN or add a working copy later on.
This may be a silly to suggest, but Nautilus on Ubuntu Linux has mounting of network drives, and ssh repositories and such built it very nicely. I used to edit code using this method with gedit. As for the mac, I would say that Transmit with its MacFUSE plugin is probably what I would use. What error are you getting?
I think your best option is to mount the remote working tree locally so it appears like a network file system, that way you can use all your exisiting SVN tools.
As to how you can mount the remote working tree, that's depends on how much access you have on the server and whether you can install new software on it.
You can use sshfs (included in macfuse), that's probably the most straight forward to setup, no commericial software needed.
VPN + LAN Sharing
You can VPN to the server (using OpenVPN, PPTP or LogMein Hamachi) then mount the working tree as a Samba/NFS share locally.