There should be no good reason why you can't access the pfSense webgui from your laptop connected via wifi. Here's what I would do.
Connect the pfSense WAN interface to one of the switch ports on the WRT54. Set the pfSense WAN to use DHCP, it will then receive an IP from the WRT54. Set a firewall rule on the pfSense WAN interface to allow access to the webgui on WAN, so: source any, destination WAN address, port 443 (or 80 if your using http). Done, you should now be able to access it via a wifi connection to the WRT54.
It sounds like you're trying something more complex, is there some reason for that? As you say it's good experience to try some thing new.
Steve