Actually you have two different issues to deal with, 1. installing a bootloader onto a blank 1284P chip and 2. modifiying the arduino IDE to be able to function with a 1284P based chip.
The first is rather simple if you use Nick's great sketch that runs on any arduino board and will interrogate the target chip and if it sees that it is a 1284P will install a booloader onto the chip. Although I'm not sure what 'flavor' bootloader it installs.
The second task is to download and install the needed files and folders to modify the arduino IDE to allow it to work with the 1284P chip. Member maniacbug has done all the hard work and has all the file/folders needed to allow that to happen. However the detailed step by step instructions to follow are beyond my ability right now, I just kept fuszing around with it rather randomly and with most difficulty finally was able to get the IDE to work with a 644P chip, as I'm still waiting for a 1284P chip to show up in the mail. So it's certainly doable but all the steps needed can be error prone if it's your first time and no one is around to hold your hand. ;)