Geoff,
The missed approach depends for obstacle clearance on being started from the MAPt; revising the procedure is not a simple matter and it must obey the basic principles of the regulations of the State in which it is being undertaken.
Providing the VNAV that results in the RNP APCH or RNP AR APCH, or the process that results in the non-precision approach overlay should not be undertaken in an ad hoc way but should be part of a SMS approach to provision of CDFA (advice should be sought for the non-cognoscenti).
There is a substantial difference in the provision of the DA(H) or MDA(H) - as examination of ICAO Doc 8168 Figures I-4-1-2 and 3 will illustrate. As you will see from these diagrams (which I have sent to you), the OCA(H) will have height additions which result in the 'lower limit' and 'margin' which are used in the provision of DA(H) or MDA(H). These additions will include both those mandated by the State's regulation and those added by the operator in providing the OM limits.
You might/will find that not all States (EASA, FAA etc) use the same process, it might be difficult therefore to provide a simple set of rules. As a first step you could examine any CDFA profiles that have been provided in your operator's OM - this will give you a good idea of how much progress has so far been achieved towards the provision of CDFA procedures.
Jim