Custom Triggers for keeping a new host in drain mode until desired

Current functionality allows for new host to be held in drain mode until Nerdio finishes installing apps. Would like the ability to hold a machine in drain mode until a particular condition is met. For example; installing a set of applications outside of Nerdio and placing a flag file down when finished. Request to run custom PowerShell code with a while loop waiting for the flag file to be present and deleting flag file once found. Would need ability to abort custom wait after a specified time (60 minutes, 120 minutes).