Hi again, I have an update!
Firstly, we do have opt-in geolocation by the browser, but it wouldn't persist to the URL. This might be your best bet if you're on the go.
I hope that's helpful! Thanks again for writing in. We're currently working on improvements to the Trip Planner, and we expect to continue working on an improved trip planning experience over the coming months. Please don't hesitate to reach out if you have any questions in the future.
All the best,
Developer@MBTA