SSL VPN solution through any proxy/network

Hi,

I'm looking for a vpn solution which works fine through any kind of proxy-server.

Problem is that a company I know has its employees working for their customers in different kinds of situations but they need to be able to access mail and files of their own company.
They used openvpn which worked fine but now an ISA reverse proxy is causing problems.

I know Juniper has an SSL VPN which offers a webbrowser f.e. https://vpn.company.com, you download a plugin and you can open applications or make a vpn.
I would like to look at that kind of solution, but budgetwise. Openvpn still needs to be able to make a seperate connection parallel to the browser whereas the Juniper SSL VPN plugin is a part of the browser and will never have connection problem.

Do you know which solution could offer this (budget-friendly)?

Thanks for your input!
J.