Hey Lance,
From my testing, there is no way to start/stop the timer from an OnScreenLoad.
But what I did find on the FieldServiceMOB_WorkOrders screen we are using a "Call WebService" action for the Start Timer or Stop Timer button objects on this screen. If you click "edit" for either the Start Timer or Stop Timer button objects, you will see the "Call WebService" action used here, and you can view the details.
What you can try is an OnScreenLoad ~> Gain Focus on a Start Timer button obj - that makes the web call to Start Timer. I have not tested this on my end, if you try this route please do follow up.
- Mortaza