You can define your Fext to include the “correction” (for your application) to get Vrel. Have you looked at that approach? The forces input to the Aerospace block are not defined as only external forces.
This derivation has been gone over with multiple customers from major Aerospace companies which have agreed with the very generic representation of variable mass in Aerospace Blockset. There are other applications in which mass is dropped from the system and block is generic enough to handle that case also.