Hi mate, having done both jungle courses and taught out there, the pre requisite is min rank of full screw, min qualification is scbc. The only other arms I've ever seen on the course is the RAVC lads who are doing the tracker dogs postings.
The actual jungle courses are run in Brunei by TTB, Belize, which as you rightly say has wound down now, was purely a trg area for sub units conducting jungle training. The Dutch marines have been using it heavily. To my knowledge the only instructional courses there have been the international jungle warfare advisors course.
If you want to get to the trees that bad, bid for the Ground sign awareness course in Brunei which lasts two weeks mate and is open to all arms.
I did 3 years in Malaya and Borneo in the 60's but all I can remember is heat, Tiger Beer, more heat, Bugis Street, Char wallahs, Kai Tais, Chinese whores, pennicillin and fighting with Aussies. Did training at Johore Bahru rather than Otterburn though. Need any tips, I'm your man.
The RAC ran a Jungle warfare course in Malaya in 1973, many of my friends (under 18 and too young to go to Ireland) went on the course and loved it. By this time I was age 18 and was stagging on at RUC Belleek in Co Fermanagh instead of plodding in the Jungle and enjoying R+R in Singapore, but I'm not bitter!!!!!
It was a one off course and don't think it was ever repeated.