I would really like the ability to configure Load-Balance policies based on the source port of the data.

The reason for this is to provide the ability to direct all Skype traffic onto a specific WAN connection. Within Skype I can define the application port as 12345.

I am using a 2920 but cannot see any way of achieving this with the current firmware.

Source port is pretty much useless for specifying traffic (because it's random 99% of the time), with load balancing, you can only specify the outbound traffic, which you do using the Destination port.
If you want to load balance WAN traffic, you've got to do so by controlling which IP the remote service connects to.

I guess asking for a feature purely to support one application is perhaps a little too optimistic