I traveled to Japan and Canada during the orchestra break and couldn't really bring the horn with me with all the baby stuff in the plane (I never thought there would be so much!)
So I decided to try just the mouthpiece for three weeks to see if it really makes a difference and the verdict is: it works!
I played about 30 minutes/day of Kopprasch with the mouthpiece only almost every day. It's not easy to play all the notes in tune but a did my best to play the etudes as accurately as possible.
When I started to play on the horn again after three weeks, the sound wasn't quite there but I had almost as much strength and endurance as if I hadn't stopped playing. The sound came back after a few days of practice.
Normally, it takes me one day of practice per day off to get back in shape. So I would have needed a couple of weeks to be in concert shape this time but with 30 minutes of mouthpiece practice almost every day, I was ready to play a decent performance after 4-5 days.
So yes, buzzing your mouthpiece on days off can help keeping your chops fit while you're away from your instrument but you need to be motivated and consistent. Also plan a few days to work on your tone and flexibility with your horn before your first gig.
I am associate principal horn of the Singapore Symphony Orchestra and the author of the progressive methods. I'm happy to share my experience as a horn player and teacher with you.