The answer is in line 231 and 239 of src/bitcoinjs-lib.eckey.js
Of course you need to change all the other lines above and maybe expand the JS to change more of the HTML.
You can choose your multicurrency selector to work on specific keywords or just allow free prefix selection.
This is of course pretty useless (you can use vanitygen and the coin client anyway) without Mycellium or a similar paper-aware mobile app for safe cold-storage spending.
So we need to fork Mycellium and their server infrastructure