I have made a short how-to video on how to flash one of the many awesome roms (made by our awesome rom-makers) onto a device that doesn't have a power button. I have done it several times and as the question keeps getting raised, I thought I would show some proof.
Some advice before the video.
DO THIS AT YOUR OWN RISK!!! I'M NOT RESPONSIBLE FOR ANY DAMAGED OR FRIED PDAs
I have done this myself on an I-mate (Himalaya). It took some trial and error but I am now running WM6.1. In my case though, I had to completely remove the power button from the device. The previous owner dropped it on its head, and I got it for free, unfortunately most of the top is missing, including the power button.
SO, on to how I did it. I took a paper clip (metal) and put it about 1cm down, slightly to the right of where the power button used to connect to the motherboard (in line with the text section of the Contact button on the top left of your XDA). Make sure you have turned your XDA/IMATE on before doing this. You'll find that when you find the right area, your device will turn off, or the screen will go dark (like holding the power button for a second or giving it a quick tap) or it will flash on and off.
If you manage to hold it there for the right period of time, while pressing the other bootloader buttons, then you will be able to enter bootloader.
This took me close to an hour to get just right, on both occasions I have done it, but it eventually worked and you eventually get quicker (5 minutes for me now).
My IMATE was useless without me updating the rom, so I had no worry about frying it if I did something wrong, that is why I was OK with playing around with its insides. BEWARE that you MAY DO DAMAGE to the device.
Here is the video.
Cheers! Hope this helps some people out there get the most out of their Himalayas again.