WebDAV was my first thought, as well. I don't know anything about Windows, but it's generally an alright solution for crossing slow networks, compared to actual "mounted" protocols like NFS or CIFS.
Between OpenVPN and having the disk encrypted, you're probably hammering that server's CPU for large file transfers over the VPN. If you're using SSL in Apache for the WebDAV host, even more so. Check out the CPU load during a couple of concurrent large transfers and see how hard it's getting hammered - if it's bad, you may want to consider using a cryptographic coprocessor, like the ones Soekris sells for under $100. Use AES and make sure all of your software is compiled and configured to take advantage of the crypto card.
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