Are Nested Configurable Subsystems Supported?

I am using MATLAB V 7.13.0.561 with Simulink V 7.8. I am exploring using Configurable Subsystems as a means of maintaining model variants.

Is it possible to use a nested Configurable Subsystem? I currently have a main model file with a configurable subsystem with 3 subsystems to choose between. I would like to make one of these 3 subsystems a configurable subsystem.

For example, I have subsystems A, B_1, B_2 and C. Subsystems B_1 and B_2 are in a separate library with a Configurable Subsystem block that has both B_1 and B_2 selected under "List of block choices". When I copy the block config_subsys_B into another library, I get a link to the B library. This library now contains subsystems A and C, in addition to config_subsys_B. When I bring a new configurable subsystem block into this library, only A and C are available choices under 'List of block choices'. Is there a way to make config_subsys_B a selectable choice?

I realize I could achieve the same functionality I am trying to get by bringing all four subsystems into the same library, but I'm just looking to find out if this is possible.

Thanks!