USB-C DRP can working for your project. There are many reference solution for your refer to.
Thanks for the quick response and ref. design links.
While both of the links demonstrate the capability of using the CCG3PA for DRP applications (both source/sink through a USB-C port), I don't believe either discuss using the port as a source AND sink at the same time.
The south chip link clearly states that the "portable external battery ... stores charge when connected to a charger and later delivers charge to an attached device".
The Qorvo link looks more promising (perhaps the USB PD controller can manage what I am looking for) but it is not clear to me whether bidirectional, simultaneous power paths are possible.
To return to my original question, it seems that VCONN, can be used as a supply to the peripheral after the CC has identified polarity. See the following link: https://www.silabs.com/community/mcu/8-bit/knowledge-base.entry.html/2016/09/26/what_s_the_role_ofc-kQYe, number 2.
It seems in either role, the host device should be capable of providing a small amount of power to the peripheral.
Can anyone confirm that the I/F can be configured so the PD sink can be the VCONN source?