ok, I might try this and see what IR LED that I can place in discrete locations to light up the scene within the camera parameters.
Can I trigger the IR LED by the camera by using "IR flash"(that I can build) on the camera that will trigger the other IR just like the external flash used with a camera?
IR LEDs come in many sizes and power ratings and can be focused and guided just like normal light. They will likely use more power than your camera will if you're trying to get it bright enough to light at much of a distance or a large area but fortunately they only need to be on for a second or two at a time.
Yes, you could make a sort of IR flash, just design the LED or LED array to be triggered by an external pulse and have a programmable or adjustable on-time for each trigger pulse. Use a photocell so it is only used when it's dark and saves battery during the day.The details are left up to your power source, trigger pulse source and timing needs.
You can find 10 watt IR LEDs for about $20US which should do well, just remember the human iris doesn't clamp down in response to IR so don't stare into one of those things just because it doesn't 'look' bright to the naked eye.
Another possible option to light up a large area would be a Xenon flashlamp (like a normal camera flash) they output a fair amount of IR, you could use a broadband IR filter over it to eliminate most of the visible light. I cannot recommend this as I have not tried it, but certainly worth investigation. The down side would be that you have to charge the flash circuit before you took the picture, so you would need to automate some of the sequence other than just a basic timed camera trigger. Leaving the capacitor charged all the time would eat your batteries.
You haven't mentioned what type of camera you are using which would affect the details of this project a lot(Will it run on 3.7 volt, or do you need 7.4?). Is it going to just be a cheap no shutter digital camera, or are you looking at placing an SLR up there? What are the current requirements of the camera? This information will affect your battery size requirements.
It sounds like a fun project, there are a lot of existing 'DIY outdoor time-lapse camera' projects out there and they all seem quite different to fit people's specific needs.