Conceptionally, the MUD-URL really belongs to a class of devices, and not so much the connection profile. That is, when you have a specific IoT device, then this device probably should use the same MUD URL on all profiles (at least by default).
We already have a mechanism for that: global connection defaults. Use that. This allows a vendor drop preinstall a file "/usr/lib/NetworkManager/conf.d/10-mud-url.conf" with
Note that we introduce the special "connection.mud-url" value "none", to indicate not to use a MUD URL (but also not to consult the global connection default).