It's not illegal, in a G-reg plane.
You can descend to any height for the purpose of a landing, on an unpublished instrument approach.
IO540
Can you clarify this comment regarding unpublished approaches in IMC
If the runway at destination is on a radial from VOR, the cloud base is 1500 ft and the MSA is 2000 ft I would have thought that you should not descend below MSA in uncontrolled airspace whatever the level of ATC at the airfield.
I thought that if there is no published procedure at the destination airfield that one should cloud break using a published approach at an alternate airfiled and then transit VFR in VMC below cloud base if safe to do so. If this is not possible then one cant land at destination airfield.
This comment suggests that it is legal to use an unpublished approach to cloud break below MSA.