Is there a way to have an wireless network that is isolated from the LAN when we are not using the 2930 as the DHCP server?

We've tried the setup and the wireless clients are connecting but are not being assigned an IP. I presume this is because they can not communicate with the DHCP server.

We'd like to setup a visitor wireless network but ensure they have no access to our local network.

You need to use DHCP relay for that on the LAN - General Setup page, put your main network's DHCP server in there and the 2930 should then forward DHCP across the VLAN to your isolated network.

Does this cover it?

http://www.draytek.co.uk/support/kb_vigor_multiplessid.html

I guess you would need DHCP enabled.