What actions are you looking to perform with the PowerShell, over the built-in deployment logic? Do they need access to the extracted database scripts?
If not, then you can package up some custom PowerShell using RgPublish, and add that as a step, that can be reused as needed.
If they do need access to the database scripts, then this would need to be done using the sqlCI command line tool / build scripts, that are part of the automation pack we mentioned:
http://www.red-gate.com/products/sql-de ... tion-pack/
Using custom PowerShell that has access to the database scripts is not possible from the SSMS add-in yet. It's something on our list to address. How big of an issue is this for you?