Often go to Manila and have done some (a very little admittedly) riding there.
I think some of the security points are a bit overblown – yes, you will stand out and people will look at you, but you just need to be sensible as you would anywehere – avoid the dodgy bits, don’t take anything unnecessarily valuable etc etc – I don’t need to tell you that.
Traffic is abysmal – as above, don’t try to ride in it – you need some sort of vehicle to get to the riding, in my opinion. Pollution is vile as well. That said, we we borrowed bikes and went riding up to a country club – Timberland in Marikina. It was a good haul up to it and once we got there, there were a lot of bikes, including – to my suprise – an Orange 5. Lots of cheeky trails, which was a lot of fun. No real concept of rights of way so we just explored.
There is an mtb “scene” in Manila and even a couple of bike shops, but like most things it’s a little “make it up as you go along”.
Good luck! It’s a beautiful country.