No, thats just stupid. A good solution would be a guest WLAN that is, for example, on a separate VLAN. There is no reason to force people to use the slow and expensive cellular network -- both your employees and the people who
actually need to use the cellular network will thank you.
Also, the GP companies no-peer-to-peer-at-all policy seems... not well thought out, even thought it might be a pretty common policy. That an application is p2p has nothing to do with wether or not it can be trusted. (If the application wanted to steal data, wouldn't it be easier to just go for some central dump server?)