This application note from Maxim talks about how you can wire up multiplex style two digit displays to the 6951, which you ordinarily couldn't do because of the charlieplexing matrix.

This might work nicely for the clock because it has three two-digit groups, and on newer versions they're separated by colons. With the addition of three BAT54C diode arrays, this could be done instead with three two digit modules.

The only real motivation would be if that wound up reducing the cost. We'll have to price it out and see, but it seems likely to me.

The same thing applies to the Raspberry Pi Clock too.

EDIT:

Turns out that they do have .56" 2 digit modules, but they don't use matrix style wiring. They have 18 pins, which is 9 pins per digit - one common cathode and 9 individual anodes. So switching to these modules (which are about the same price as a single digit module) just requires a new board design - no (real) schematic change.