You appear to have missed the point Nick and we made earlier, we did not say 64MHz internal on the K22 is not possible, it clearly is because the 20X2 already uses that.
However the 28X2 is intentionally designed to optionally use an external resonator as the primary, as many people prefer the accuracy of an external resonator/crystal. Therefore PLL cannot be used on internal oscillation with *the intended 28X2* configuration - primary FOSC external, secondary internal.
In other words we can't always please everyone, at design we had the option of
-up to 64MHz internal but no external oscillator options at all (as used on 20X2)
or
-options of both the external with PLL oscillator (up to 64MHz) and the internal oscillator (without PLL, so max 16MHz internal).
We choose the latter for the 28X2, so if you want 64MHz on a 28X2 simply solder in an external 16MHz resonator.