One thought we've had is to build a SA602/SA605-based fundamental mode
Colpitts oscillator (with two capacitors and the crystal) and then tap
into the mixer output, leaving the RF intput disconnected. If the
mixer does not mix with anything, we will get an amplified/buffered
13.56MHz signal. Would this not work?

Also, I am not clear how to chose the capacitor values for the
Colpitts oscillator - there is no guidance in the application notes. I
think these values are not critical? Our crystal frequency is
13.105MHz.

Ben