Crab,
There could be several reason why no-one has replied to your question on descent below MSA (although I did several months ago - probably on a PM) and that is because the regulation establishes that it can be done but does not spell out the method of compliance. It is left to the operator to submit a proposal. Any reply could therefore reveal the company of the poster.
Firstly bear in mind that the MSA (LSALT) over the sea is 1,300ft (usually rounded to 1,500).
Apart its obvious use for an en-route descent - e.g. to avoid an instrument approach to a complex environment (where the approach and go-around directions are not clear of obstacles); it can be used as a fall back procedure in Coastal Aerodromes procedures; and a descent in compliance with Limited Icing clearances. Here is the content of the rule:
JAR-OPS 3.365 Minimum flight altitudes
(See IEM OPS 3.250)
The pilot flying shall not descend below specified minimum altitudes except when necessary for take-off or landing, or when descending in accordance with procedures approved by the Authority.
Any applicant would have to submit a safety case establishing the extent and limits of the procedure together with requirements for equipment; accuracy of height would require the RADALT and obstacle clearance in the descent sector, the airborne RADAR.
The more specific of any application would be that submitted by a SAR operator for whom limits would be directly associated with their operational constraints and the demands of the task.
Such a procedure would be in the Operations Manual (or, for the SAR unit, the SAR Appendix).
Jim