The v4 USRP has a fan header next to the power jack. I don’t know what
the recommended solution for v3 USRPs is, although I suspect it’s along
the lines you mentioned. 
Some v3 USRPs already have a power lead
takeoff for an oscillator sub-board on the bottom of the USRP; I suspect
the fan leads could attach to the same place. Hopefully there is
filtering enough that the fan motor would not butcher the oscillator
signal, but someone else will have to speak to that.
Best,
Mike