Animate I wish I could offer some javascript advice but i can't unfortunately.
The way I got around the problem was to increase the speed of typing fast enough that it generally fooled the eye. it wasn't perfect but it became acceptable.
The only way i could see hacking around it for per letter accuracy might be to set up a series of text updates each with a particular word you want underlined actually < >'d so that each letter is underlined so..
hmmm another thought (i haven't used this in a long time) but could you possible underline each letter next to each other and them appear underlined.. or would it just look like a bunch of letters with individual underlines under them? i imagine it would since they are being individually underlined.. tricky.