I think that if you wanna see the schematic you'd better go to SoC development platform, something like "evalue board", which comes with schematic and such a detailed hw stuff. "Carambola" boards may be interesting.
No, the Carambola doesn't give you the module schematic either, only the uninteresting connectors and passive stuff...
About WR703N i think it is a toy, poor ram (32Mbyte), poor flash(4Mb), hardcoded uboot's env, no good at all! Also only 2 usable gpio.
You can find some hacks to upgrade the RAM to 64MB and the Flash to 16MB, or buy some already upgraded for you.
The hardcoded environment is really not a big problem, since you don't spend your life in there:)
As for the GPIOs: Linux is far from real-time, and beside blinking a LED with these GPIOs, it is not very useful. The best thing to do is to connect it to a microcontroller to delegate the real-time GPIO control. In this regard, the Arduino is a very good choice IMHO.
There are cheaper alternatives to the WR703N, like the TopLink TOP-AP01:
Only $7.5 in a 8MB/32MB configuration...