Dring the Mack Brown era:
I do not remember anyone making a decision to sign with Texas on signing day. There have been very few make their decisions the week of signing day with Frank Okam making his decision in 2004 the night before. With earlier commitments in February rather than the summer, there has been even less last minute decisions.
Now, by this time there are very few additions before signing day. Among the Texas commitments, 95-99% sign. Almost all who decommit do so during the summer or fall and not at this time. One of the only ones who did not was Perrilloux, but the way he was talking, he was not really committed even though he was listed as one. There were at least two others in previous years who were listed as commitments, but did not sign on signing day for one reason or another. Other schools have recruits who hold out and sign signing day and some who you would expect to sign with one school end up signing with a different school. I do not remember this happening to Texas on signing day.
Saying all of this, I do not expect Texas to add many more if any players. They could add one or two more that are still considering Texas that the Texas coaches are still recruiting, but the odds are against them adding many more recruits.