Colin - to cut through the clutter, I have read hundreds and hundreds of posts and thousands of comments on visa related boards and groups...
I have not read of one report where at a Thai Border Checkpoint / Border Crossing that an I/O or an Immigration Officer at an airport asking any foreigner who comes to Thailand holding an O-A Long Stay Visa or someone who is entering on a 90 day O Visa or B Visa or anyone on an Extension of Stay based on being over age 50/Retirement to show 20,000 Baht cash or any other financial evidence.
These report only apply to people who come to enter Thailand on Tourist Visas or Exemptions for not having a visa or those who enter on Visa on Arrival.
It does not seem to apply to those on Multi-Entry Tourist Visas who are doing an out and back in to get another 60 day stay under the terms of the METV...
That rule is a Philippine Government Rule that the Thai Consulate enforces and as does every other foreign Consulate in Manila. It came from an effort to stop Philippine workers from being lured to jobs and sometimes virtually held as hostage in a foreign land such as Saudia Arabia with no money to get home. This measure was instituted to prevent economic slavery of Expat Philippine workers. It is not actually a Thailand visa rule but a rule emposed on all countries by the Philippine Government