I live in Shanghai now but I just moved here from Bangkok. I'm not sure where you are going to be in S.E. Asia but here are three links for info:
Bali- haven't riden with these guys yet but the come well recommended from a friend and I am planning to ride with them in June for two weeks. http://bali-trailblazers.com/index.asp
S.E. Asia- I have riden with them three times, twice in Thailand and once in Vietnam. Both times I was highly impressed with the attention to high quality trail selections and overall quality. Struan, the Bangkok manager, is an excellent rider and if you are up for it he or his staff will lead you through some amazing areas. http://www.spiceroads.com/
Bangkok- This is a local group that rides once a month. There are all types of riders in this group, racers-newbies. If you want a cheap way to see an area around Bangkok over a weekend this is a good option. http://www.bangkokbikehash.com/
Hope this helps and I'm sure already no this but most areas in S.E. Asia at this time of year are extremely hot. Riding still happens but just be prepared for 105+ temps.
Yeh i'm S. Korean. And if you want to find all the good places. Here's a link to some of the "actual" mountain bike related shops. http://www.mtbk-adventure.com/bike_shops.php
The best one was O.D. Bike. They had the sickest stuff there. Nicolai, Santa Cruz, Foes, Intense, Deemax's, etc.
So you could ask the employee's there about trails and etc.
Yes, S Korea, although not SE Asia, has some world class tech DH and FR. April should be great. July not so great (rain). MUST hook up with local residents or live there top spend time looking. Seoul in particular has great descents on Yongma San and Namhan Sanseong, although most any mtn in the area will have some kind of steep rocky track you'll have to carry up a lot of.
Bali has great smooth singletrack descents down the north face of Volcano Batur to black sand beaches, and then go snorkeling. 1900m vert, 1 hr DH race speed descents. Monty. Lots of stuff to explore. The link someone posted above is all XC and the company doesn't seem to have found/commercialize these descents but they're in the same neighborhood. Start at north rim of Batur crater. You can hire a car and driver (nearly same price as just car) and shuttle 95% of the way up.... or if you're a XC nut you can ride up.
Hong Kong: all singletrack illegal for bike last time I checked but locals apparently have some pirate tracks...
Malaysia - I hear has a decent DH scene. Search here on RM for some of them; they're around.
Taiwan - monty trails, lots of vert. Google Formose Fat Tire. Connection swith S Korea's filthy scene.