We've had drilled poly lines installed above soil/below mulch for almost 20 years without any special precautions or problems with freezing. Being on the surface, believe enough water drips out when turned off for expansion space when any remaining water freezes. Might be different if buried shallow, especially when the soil is wet.

We had a few tiny sprinklers on a circuit for several years before switching to more xeric plants when some trees came down. Don't recall any freeze problems with those either; because they were tapped into an underground sprinkler circuit and above ground in a higher part of the yard, am pretty sure they self drained through the lowest in-ground head when turned off. There was an isolation valve which was left open in the winter so it could do this. Both the surface sprinkler head and tube systems always suffered most from our own heavy-handed trimming, weeding, and cultivation.

Thank you!

I still haven't installed mine, but the master gardener's demo garden used it under the mulch too and never had an issue.