There are many markets to choose. Check out the CBOT interest rate products, as well as the Eurex interest rate products.
The CBOT markets are doing quite a bit of volume and are quite active. Plus, at $31.25 per tick there is a lot of bang for the buck.
Browser based systems would be a viable candidate for this type of configuration. Any VPN based solution will most likely demand some port changes on the client firewall. PATS may be the exception depending on what ports are available.