This probably qualifies as obvious:
The fundamental assumption of most firewall design is that the upstream environment is hostile and the downstream environment is benign, so it's safe to let anything through from downstream. This assumption is especially dangerous when the downstream environment is wireless because of the difficulty of controlling access to a wireless node. But IMHO there is nothing inherent in the design of e.g. ipchains (the only implementation I know much about) to _require_ it to be asymmetric.
As usual, you need to begin your design by deciding
a. what goals you need to accomplish with the AP;
b. what costs and risks those goals are worth;
c. whom you will allow to connect to this AP;
d. how you will identify them, both before and after they connect;
e. what you will let them do once they are connected;
f. and how you will respond when they try to do something else.
And depending on items b. and f. you may have the further task of trying to persuade management that this AP is really not a good idea.