Differentiaing Wirless and LAN Clients through NoCat

I've got the NoCatSplash package apparently working on a Linksys 
WTR54G, using the latest beta of DD-WRT (23).

It isn't really working as I expected. I thought it would force 
Wireless clients to the splash page, but let LAN/ethernet connected 
clients straight through, so that staff at the coffeeshop I'm setting 
it up for wouldn't be affected. The reason why this matters is that 
I'm blocking port 25 as suggested and this will obviously cause 
problems for the coffeeshop staff.

So first of all, is this expected behaviour? Should LAN clients get 
sent to the Splash page?

Secondly, if it is, can anyone suggest a way of modifying the config 
to allow this? Would I, for example, be able to modify the firewall 
rules to allow a certain block of internal IP addresses through. If 
so I could then assign the staff with fixed IPs in a small range and 
that would solve the problem.

Or is it simpler than that? Can I use vlan functionality to assign 
all the LAN ports to a different VLAN and somehow exclude that from 
the nocat treatment.

Hoping someone has been through this already ..

Jim