AC flight number, AC branding, AC ticket number, all of that is meaningless unless AC has negotiated access for AC Rouge customers to use the lounges. Lounges can treat AC Rouge as a separate airline if they choose to (since they fly are "operated by RV").

Some airlines are good at maintaining consistency for lounge, such as when SWISS had Edelweiss Air flying for them for a brief period of time.

Other airlines such as South African Airways--they have SA Express and Airlink operated flights, separate companies but all plastered with SA flight numbers and branding--and they deny entry to customers on SA Express and Airlink to their own SA lounges in JNB.