Mostly, on HF, the FCC has chosen to limit modes of transmission and symbol rates rather than bandwidth (60 Meters is the exception, where they have limited both bandwidth and mode). For instance, you are currently allowed to use FM only on 29MHz and above, regardless of the bandwidth used. But on VHF and above, there are no legal limitations of bandwidth.
So technically, yes, if you had a wide deviation FM transmitter you could use it on 2 meters. Although as I'm sure you know this would be very irresponsible [but not illegal according to current FCC rules.]
On 10GHz and similar high bands with tons of space, it is commonplace to use wideband FM (normally a bandwidth of 200-300kHz). This allows equipment to be used with less than optimal frequency stability (which is very difficult to obtain at those high frequencies).