I haven't camped in those conditions, but I've camped in a lot of rain and I tend to believe the water sensitivity of down is overblown. Every time I've tried to wash a down bag, it is a pain to get the down wet so I can wash it.

With high humidity, I guess you are worried about the down slowly absorbing moisture from the air. I would still prefer a down bag and be sure to dry it thoroughly between trips. You could also get a bit warmer of a bag than you otherwise would to account for some degradation during the trip.

If you make the decision that only synthetic will work because of the constant humidity, look into a quilt as was mentioned above. If you don't want to make your own Ray-way quilt, I've been quite happy with my Enlightened Equipment down quilt. They also make synthetic quilts. Though you might be able to get a local seamstress in the Philippines to sew a Ray-way quilt for less out the door than an EE quilt.