Sounds like he's looking for Tasker. One of the first apps I install after a wipe.

https://market.android.com/details?id=net.dinglisch.android....

I'm pretty sure he's not going to see this reply but I thought any HNer that thought "that would be helpful" should see this app. There is likely a better way using solely Tasker but this is my first run at solving.

New Profile: On Time event: repeat every 30 minutes from start time to end time.

Add action: Net dialog: Wifi to on.

The whitelist would reside in Wireless Settings. Any AP already connected remains as a preferred AP. So, Tasker flips WiFi on and Android spots the AP.

Then, create an Exit event the Tasker profile. If Wifi Status is disabled then turn Wifi off. Tasker recognizes two Wifi states, enabled (connected) and everything else.