I have not tried what you are attempting but here is an excerpt from:
http://superuser.com/questions/43207...ethernet-ports
"Proxy STA" Wireless client mode, a.k.a. "Join w/ 'allow Ethernet clients'"
I believe this mode first shipped with the 802.11n AirPort Express (early 2008 to present). You'd use this for the same reasons as normal STA mode, with the additional ability to bridge wired Ethernet devices onto the network.
Because the 802.11 spec does not allow normal wireless clients to transparently bridge traffic, the radio in the AirPort Express has to do extra work in this mode to sort of clone the MAC addresses of any wired Ethernet devices it sees connected to its Ethernet port, and fool the upstream AP into thinking those MAC addresses are all separate wireless clients that have all joined the upstream AP (that is, it has to perform the 802.11 "STA" role on behalf of those non-802.11 devices, thus "Proxy STA").
This mode supports:
Bridging Ethernet clients in
Sharing USB printers
AirPlay/AirTunes output on AirPort Expresses.
This mode is supported on:
AirPort Express 802.11g (2004-2008)
AirPort Express 802.11n (2008-present)
(
Which is to say "All AirPort Expresses, but not Extremes or Time Capsules)