I'd also thoroughly recommend the stretch of road alongside this spot, and it is (or at least was a few years ago) possible to get there on public transport, and stay in a cheap place a few km away, and either walk or get a cheap lift to the best stretch for birds. I'll try and add a link to how we did it and what we saw...
http://www.birdforum.net/showthread.php?t=100901&highlight=bristol+kagu&page=37 scroll down to post #904 on that page for the post. More up and down that thread for elsewhere in Borneo
below based on my experiences in 2011:-
you can stay at a basic but nice place called the gunung alab resort. this is about 5km uphill (direction of KK) from the rafflesia centre (very close to the pass).
to get there you can get share taxis from kota kinabalu to tambunan which will drop you off. These leave from the south end of the minibus station in KK. It took me a while to track these down as everyone assumed i was saying "semporna" when i asked for "tambunan" - don't ask me!
I was lucky to get the last space in a car but you should probably factor in the possibility of a wait.
when you are there it's easy enough with plenty of time to walk downhill to the good roadside birding a few km either side of the centre, i wouldn't recommend walking back up though. I was lucky that the resort owner (who was really kind all round) came and picked me up for free both afternoons i was there. She also promised to drive me down early in the morning but overslept so i walked instead!
good range of hot meals at the resort, also you can buy nasi goreng etc. from the cafe at the rafflesia centre. should probably take your own breakfast, snacks, water etc. though.
getting away by public transport may be more difficult depending on where you want to go but the resort owner will probably be able to help. i was going to kinabalu NP so she kindly gave me a cheap lift to tambunan and sorted out a share taxi to ranau, although i had to wait several hours before it actually left!
one drawback of birding the rafflesia centre roadside is you will be honked by every 2nd vehicle that passes, no doubt all meant in good fun but very irritating after a while!!
cheers,
James