Speaking as a simple controller, if I recall correctly, the 'terrain protected' area around a hold is for an aircraft established in the hold (i.e. over the beacon and turning onto the outbound leg) - so I wouldn't descend until that point. The protected area may be varied from the standard volume in a terrain rich environment by, for example varying the length of of the hold or the speed at which it is to be flown - so you would want to check such details before descending below MSA in IMC.
You mention 'letting down' in your post. No doubt you are aware of the legislation that SkyCamMK referred to - in the UK it is a bit complex but essentially to do an instrument approach without an ATC service is not permitted.