Well, if you wanna know truthfully, I don't think it's possible without someone that has knowledge of modifying the actual models and creating one for every race that has exclamation point over their head, which is triggered off an OnUpdate, and then I'm pretty sure that would be damn near impossible to do. Good luck to anyone willing to take this on, but personally, I don't think anyone will take the time, if it is possible, or won't do it, seeing as how I think it's impossible. Sorry.
Here's an idea: Replace the Hunter's Mark graphics and sound! =P
I know that's probably not even possible, I don't know how the models and textures are stored... but I thought I'd toss out the idea. It wouldn't provide the functionality you wanted either, but it'd still be fairly amusing. :wink:
Oh, and by the way, does anyone have that alert sound? I searched for it a good while back without finding anything. Thought it might be fun as an sms ringtone. Oh, yeah, and the Codec call sound for a regular ring tone!