Yes, it is possible to define vintage-dependent subsidies on commodity flows, directly proportional to the flows of each vintage.
But you seem to be asking for subsidies that are proportional to some other costs (10%, 20%)?
You can model vintage-dependent subsidies proportional to flows by defining the technology vintaged. You would just need to define a new commodity for such subsidies.
For example, define a new commodity with the name "SUBSIDY" of type ENV, and define FLO_EMIS(r,y,p,COM,SUBSIDY,ANNUAL) = X. Finally, define FLO_SUB=1 on the SUBSIDY flow. This results in a subsidy level X for the flow COM and vintage y, for the lifetime of vintage y (it will be vintage-dependent). You don't need to add the SUBSIDY commodity in the topology.
For non-vintaged technologies, it is not possible to define vintage-dependent subsidies on flows, only on the capacity. Subsidies on the capacity can be defined by defining a NCAP_COM(r,y,p,SUBSIDY,OUT)=X and then putting a FLO_SUB=1 on the SUBSIDY flow. In that way you can always define a vintage-dependent subsidy proportional to the installed capacity of that vintage, for its lifetime.