What is the missed approach procedure when initiated below MDA/DA/DH?
When would you go missed if below MDA/DA(H)?

You're only allowed to operate below the MDA/DA(H) if all of the following are true (14 CFR 91.175(c)):


You can land using normal descent rate and maneuvers
You have at least the flight visibility required for the approach
You have at least one visual reference from the list (not just the approach lighting system)


91.175(e) says that you must go missed if those conditions are not met and you're below the MDA, at the MAP, at the DA/DH, or "at any time after that until touchdown", i.e. below MAP/DA(H). Essentially, if any of the conditions in 91.175(c) are no longer true, then you have to go missed immediately. Practically, the most likely issue is seeing the runway environment, but you should also go missed if your approach is unstable, for example.

On an ILS approach specifically, that would most likely mean that you saw the approach lights at DA/DH (or above), followed the glideslope down to 100' above TDZE, but then didn't see any more visual references. That should be very unlikely if you really do have the flight visibility required for the approach but I'm sure it could happen somehow, e.g. there's ground fog in the wrong place at the wrong time. Or something else could go wrong: the runway lighting completely fails because of a power cut, or there's an obstruction on the runway.

Is obstacle clearance assured if going missed below MDA/DA(H)?

No. The Instrument Procedures Handbook (chapter 4) says several times that if you go missed below MDA/DA(H) then you have no guarantees and it gives some general advice on what to do:


  If the aircraft initiates a missed approach at a point other than the
  missed approach point, from below MDA or DA (H), or on a circling
  approach, obstacle clearance is not provided by following the
  published missed approach procedure, nor is separation assured from
  other air traffic in the vicinity.
  
  [...]
  
  In the event a balked (rejected) landing occurs at a position other
  than the published missed approach point, the pilot should contact ATC
  as soon as possible to obtain an amended clearance. If unable to
  contact ATC for any reason, the pilot should attempt to re−intercept a
  published segment of the missed approach and comply with route and
  altitude instructions. If unable to contact ATC, and in the pilot’s
  judgment it is no longer appropriate to fly the published missed
  approach procedure, then consider either maintaining visual conditions
  (if possible) and reattempt a landing, or a circle−climb over the
  airport.


This part is directed at pilots using night vision systems, but it looks like good advice for any pilot considering going below MDA/DA(H):


  If a pilot initiates a go-around at a point below DA/DH or after the
  MAP, obstacle clearance is not necessarily provided by following the
  published missed approach procedure. Prior planning is recommended and
  should include contingencies between the published MAP and touchdown
  with reference to obstacle clearance, aircraft performance, and
  alternate escape plans.