Interface determination based on a custom field on payload

All,

We added a custom field (a flag) to a standard enterprise service TransportationOrderRequest_Out hoping to use it for routing in PI, i.e. determine a unique interface / mapping if this flag has a certain value. However it turns out there is a constraint that only standard fields can be used for routing / interface determination. Is this true and if so could someone advise if there is a work around?

Best Regards,

KC