VPN or RAS through ISA

[Yba]

Hi,
Current newtowk configuration is DC 2003, Exchange 2003 and ISA 2004, each
on a single box. OWA and RPC over HTTPS are configured and both work fine.
We have a number of remote clients that use OWA and I intend to make them
use RPC over HTTPS. Those clients are not members of the domain. Moreover,
making them domain members is out of scope.
I admit that I have not searched deep enough for how to configure RPC over
HTTPS for a client without bringing it over where it can connect to Exchange
over the LAN. If, ever, there is a way to configure RPC over HTTPS without
the hassle of bringing client PCs to the LAN where Exchange resides, then my
problem is solved, I just need to know how.
If, however, that is not possible, then I need to know how to establish a
VPN with my domain, from a remote client, through ISA so that clients
simulate being over the LAN. That way, I think, I can configure RPC over
HTTPS remotely. Kindly notice that a client connects to ISA through an ADSL
internet connection.
Any help with that is highly appreciated.

Thanks
Yba