Taking a look at it now...
K, what I gather: Shouldn't be too hard, just tedious. Need to know a few things about DDS files though, seeing as I've never worked with them before. Just barely got the DDS plugin for Photoshop installed, haven't had time to mess with it yes.
1) How is transparency handled in these files? From what I've seen so far, many icons are done in 2 layers: Background and the icon itself. The background layers are in icon_ss9.dds (green, blue, purple, red, orange, and yellow, most anyone who's read up on skills should know what most of those mean) along the bottom row. For the icons themselves, it looks like pure black is transparent. Hoping for a little bit of information so that I don't add an indicator to 50 icons only to find out later that I have to change every one of them.
Best way I can think of to check that is to put a pure black slash across a single icon that already uses transparency, and another black slash across a "solid" icon (one without the colored backgrounds).
2) The icon files are sorted into groups with exponentially smaller copies of the icons. I'm assuming that the only set I need to worry about is the largest set in each file, which should hopefully be the hotbutton and spellbook icons. Since I'm going to edit first and test later, could I have someone confirm that this is the only set I need to edit?
Should be easy, can do it at the same time as #1. Slash the largest version of an icon that you use with something visible. If I'm lucky (which I'm probably not), it will affect both the spellbook and the hotbar. If it doesn't affect the hotbar, it's going to be a pain and a half to do this.
Should take all of 5 minutes to check both - I'm starting work on the project now, I'll be checking the thread for updates every few minutes.