Approach Ban
Agreed,
if the RVR is within limits the approach can be commenced. Should the RVR drop below minimums before the FAF (or 1000ft AAL in the absence of a FAF), the approach must be terminated, and a missed approach flown. However, if the RVR drops below minimums after the FAF (or 1000ft AAL in the absence of a FAF) has been passed, the approach may be continued at the discretion of the aircraft commander. I should imagine this could be found verbatim on ICAO DOC 8168, PANS OPS.