Multi-WAN over dynamic multi wifi hotspots over WDS wifi network

[nranka]

Sorry for the long subject line - just wanted to make sure that I got all the relevant buzz words in there.
Ok, I had a wild idea and needed feedback to know whether the idea had any legs and other interested takers.

We have about 15 3g connection in our small office - carried by the different employees all equipment with Samsung Galaxy S Smart phones. We are paying a fairly steep monthly internet bill since all 15 our unlimited monthly packages.

Also, out office has complete wifi coverage by employing wds using tomato over linksys wifi routers.

Now I was thinking of introducing pfsense to achieve the following:

1. the pfSense machine will be the main DHCP server - none of the wifi router will be issuing any addresses
2. the pfSense machine need to determine which of the 15 smartphones wifi ap's (3g connections) are available anywhere within the office and based on certain set of priorities/rules access WAN thru the said smartphones (I'm assuming that all the smartphones - will have their wifi tethering on while in the office network); The fact that with wifi tethering on - the smartphone itself cannot connect to the office network - we are prepared to live with that.

Is the above even possible? Technically - what would the challenges be?