Ok, anyone can correct me, if I'm wrong, but if I understand correctly that manual text, the bulb mode is on as long as button is pressed in remote, so as long as camera receive the IR signal from remote, shutter is open and when it stop receiving signal, it will close the shutter. Now, the irsend program (which I use to send IR commands to camera) will only send one signal, per click (even the button is pressed longer). I think that this problem is pretty easy to solve, I just need to create own Pentax specified handler for irsend, whit use of send_start and send_stop options (I hope).

BTW, which side of camera the IR receiver is? This is important, because if it is only in front side (like in my camera), it can be problem (not for the program, but for you) to keep signal on between N900 and camera, without to be shown in photo (trust me, I know). Also can you handle the fact that the N900 has to be in same position (or at least so, that IR signal wont brake during the bulb time) from the beginning to the end of that time period you want to keep the shutter open? The button won't need to be pressed all that time, because if my solution will work, you just press it to start and stop. I just try to say that if you want to take photos longer than 1 minute, I would recommend that build some kind of holding device to keep phone in correct place (or then you had to be very patient to stand still and keep your N900 pointed in same spot as long as it is needed)