You should be able to get this with an OTA script that you can run from the workflow. The FindTestInstances function will give you the test instances in the test sets in the folder and all subfolders. Each test instance contains a reference to its Test Plan test, and the tests let you access the number of design steps that they contain.
One thing that might tricky though is if you are using Call to test and if you want to add the number of called steps to the total count.
