What variable-IF tuner IC can I use for UHF applications?
I think that most of the Zero IF receiver IC's that are available have ADC's build in, so you actually have a digital output. Even without this, ZIF receivers have IQ baseband outputs, where you have 2 analog output's for a single receiver path, which would require a IQ modulator to upconvert this into a Lo-IF approach. It's my impression that "wideband" receivers that cover 150-900 MHz have multiple conversion stages, as well as IF filters, so that they would only work over a subset of that frequency range at one time, perhaps 2 to 10 MHz within that 150-900 MHz. 

You might want to look at the Lime LM6002 transceiver part, which would give you a Digital I/Q output, which would possibly be easier to deal with in a signal processing point of view, as you have a single channel of digital information to deal with for each channel.