I have a Ruby script I run after each release that updates ST2 with my favourite icon. It's a single command from a Terminal (which I generally have open all the time anyway).
If you're on a Mac (I really don't guarantee anything else) it's here:
Before the first time you run it do this:
$ fix-sm2-icon ~/Downloads/whatever.icns
This reads, encodes and embeds "whatever.icns" within the Ruby script itself (self-modifying)
Then whenever you want to fix the icon, just do:
This extracts the embedded icon in to the right place.