Gulfstreamaviator
In the US, RNAV approaches that have LNAV/VNAV minimums usually are charted with a DA; while most VOR or VOR/DME or NDB approaches have MDA with a note "only authorized operators may use VNAV DA (H) in lieu of MDA (H)". Use of a DA, in these cases, are an OpSpec for FAR 121/135 operators and not addressed for FAR 91 operators. The FSDO will probably shrug their shoulders, if a FAR 91 operator asks for a LOA to use DA.
I don't think there is a regulatory guidance requiring adding 50', just that you cannot go below the MDA, so the 50' is a fudge to prevent going below using a VNAV to guide. While some FMS systems will honor the Altitude Selector (ASEL), others will not. Specifically, the Collins system in VGP mode will treat the VNAV PTH just like an ILS GS and NOT level at the selected altitude, in fact, procedure is to set missed approach altitude like on an ILS.
WRT the ILS, the difference is in the obstacle identification slope. A non-precision appproach, by definition, has a hard floor representing the minimum obstacle height. An ILS obstacle clearance floor is surveyed based on a slope underneath the GS, all the way to TCH or a controlling obstacle. If the mins are above 200', watch out for a close-in obstacle penetrating the 1:34 or 1:20 slopes.
To the pilot's eye, I agree that any altitude loss at a VNAV CDA missed approach would be minimal and probably accounted for. But, until surveyed going below the MDA mins is a no-no.
GF