When does an instrument approach "begin" per §135.225(a) §135.225(a) states that, "...no pilot may begin an instrument approach procedure to an airport unless—
(1) That airport has a weather reporting facility operated by the U.S. National Weather Service, a source approved by U.S. National Weather Service, or a source approved by the Administrator..."

Assuming that a part 135 pilot is operating to an airport where the weather reporting is out of service, and the pilot is not conducting eligible on-demand in accordance with §135.225(b). What portion of an IAP (if any), can be flown in this case without violating the intent? Is the IAP considered to "begin" at the IAF, IF or, for the intent of this regulation, at the FAF? Or, does it begin at the point ATC says, "cleared for the approach?" In a case such as this, the 135 pilot is needing a descent to VFR weather, since an IFR arrival (including visual or contact approach) is not authorized. Being able to descend to a the initial or intermediate segment altitude would obviously be an advantage.

If a local (or designated facility) altimeter setting is not available, terrain clearance would not be guaranteed when descending to the initial and intermediate altitudes. In that case I believe that ATC could not give a clearance for descent to those altitudes.
Beginning an instrument approach would mean being established on any segment of that approach, whether the initial, intermediate, or final approach segments, and they have received ATC clearance to do so.