You need to set a SP in the nested family.
Make the plate geometry attached to ref planes that are not the 2 from the template. Assign a SP to the distance between the RP at center of geometry and the RP that defines origin. SP will be by instance. Beware of directions for correct behavior when loaded in the host.
In the host family, add the same SP, select the plate, and in properties palette, look for the tiny button on the left. Click and choose the SP you've added. The insert point of the plate must be set to the ref level of the host.
Now you have the elevation of the plate driven by a SP in the host. Create a tag for this SP. You're done.
By Julien Benoit at
How can I get this parameter into a tag?