Synergy Mode - virtual WAN ports not available for SC WAN use?

Background:
A Balance 20X with LAN port #4 defined as Virtual WAN port #2
Establishing the B20X as the synergy controller for an upstream unit (a Transit Duo).

Observation:
When enabling Synergy Mode, only WAN 1 (the predefined/hardwired) WAN port is made available for synergy use.

Question:
Can one make the VWAN port or the USB port work as the connection to the synergized device, or is this a hard limit on which ports can be used, regardless of their status w.r.t. being WAN ports?

Cheers,

Z