can a Snow Leopard client connect to a Lion Server?

One Duo Core which is not working at the moment. I think I can install Lion on that one? Right?

You can't install Lion on a Core Duo-based system It requries Core2 Duo as a baseline. The Core Duo is a 32-bit chip whereas the Core2 Duo is 64-bit.

Then I have two G5 - none intel, which I'm thinking I'll reinstall Snow Leopard on and use them as clients.

Nope, again. Snow Leopard requires an Intel-based system. The highest you can go with the PowerPC machines is Leopard (10.5.x)

Can they connect to a Lion Server? I have a few XSAN licenses left which I think I need for OS's older than Lion?

This may be irrelevant given the above, but define 'connect' please. There are a myriad of ways of 'connecting' a client to a server. In many cases they don't have to be the same OS, platform, processor or anything else (how many web servers do you think are running Mac OS X to match your client?

OK, that makes it clearer (and don't worry about having to explain the 'policy' - I come across that every day 😉 )

If I remember rightly, it's all down to the version of XSAN you're running.

XSAN 2.2 (and later) are Intel-only, and is the standard version for Snow Leopard.

Lion ships with XSAN 2.3, and Mountain Lion ships with XSAN 3.

XSAN 2.3 (Lion) supports XSAN 2.2.x (and later) clients only, which precludes PowerPC-based systems

So, if your XSAN is running 2.2 on Snow Leopard then you can still support PowerPC clients running XSAN 2.1.x, but if you upgrade the MDCs you'll lose support for the PowerPC.

http://support.apple.com/kb/ht3755

Once you have that, there shouldn't be an issue mounting the XSAN volume on the XServe and resharing it over the network. Just bear in mind that you're basing this on 3+ year old software (XSAN 2.2) and 6+ year old hardware (PowerPC XServe).