Its cool climb, i was only one in our group to make it, was only smoker and altitude sickness never affected me. But as someone else said Kota Kinabalu is good place and we did Kalimantan river trip that was cool. Saw the pygmy elephants, wild crocs, jungle cat and orangutans (over night trip)
Very much doubt it is officially allowed but this is Thailand. We stay sometimes at brother in laws place at a Naval base, big sign saying foriegners not allowed to enter but never been an issue. But if there is a paper trail it maybe become an issue