Clientless VPN and OWA

Hi all,

My customer would like to implement OWA so their users can access email
from anywhere. I had proposed to them the Microsoft recommend
topology: put an ISA server in the DMZ and the FE servers inside the
Corp Network. They are currently running Exchange 2000 and AD 2000.

However, they want to know if there is another approach to this. As it
turns out, one of their FW supports Clientless VPN. Clientless VPN
allows their users to make a SSL VPN connection to the FW then from
there they can access OWA. This way there is no need for ISA server.

I haven't able to find a whole lot of discussion around this topic. It
looks like a good alternative, simple and less costs. Have anyone uses
this solution with great success? Any draw back compare to the ISA
method? Does Microsoft supports this alternative?

Any comment on this issue is greatly appreciated. Thank you.