I am not quite understanding the issues here. Why would you not use a Rhodebar and Rhode Island Red to improve the Rhodebar.
"It is a heavy breed which should have all the characteristics of the Rhode Island Red, it was created as a utility laying breed with dual purpose qualities; or perhaps more simply an autosexing version of the Rhode Island Red itself." harislau.info
As a means to improve the birds back to the SOP and the SOP being that of the Heritage RIR that would be the only means I would think. I am not seeing any Brussbars available. I would assume the only reason the Brussbar was used would be to integrate the wild-type e+ coming from the Brown Sussex (which are rare enough as it is).
If the qualities of the bird meet the SOP, has auto-sexing chicks that breed true, why would they not be Rhodebar. I can understand that they are project birds until they meet these standards though.