I'm glad to hear you're still interested in Wharton. One of the unfortunate circumstances of having so many strong applications each year is that AdCom necessarily must put some good applicants on the waiting list.
AdCom will generally assume that, if you choose to remain on the waitlist, you would still be eager to attend Wharton if offered the opportunity to do so. Thus, materials following up expressing your enthusiasm are unlikely to add much additional value - especially if you already have made said enthusiasm adequately clear in the application itself (which would've been the best place to have expressed such thoughts).
Also, since it is "easy" for many people to write letters reinforcing their interest in Wharton, it is not clear that writing such a letter would help distinguish you in a positive sense; in the worst case scenario, it might even cause irritation by indicating a lack of respect for a clear request (no additional material being sent). Given the volume of material AdCom reviews and receives, such additional materials might be seen in a less than positive light.
As for the former student's letter of support, I suppose they could still do so, but the time and place for this was probably earlier - during the initial application review - if even at all. At this stage, I'm not 100% convinced that such a letter of support would notably help. Some might even question why the letter of support was not provided earlier on in the process when, presumably, one would have sufficiently planned ahead to have the best possible case presented to AdCom by the deadline.
Furthermore, I believe that in several cases, the issue is not so much one of enthusiasm and willingness to attend Wharton, but a matter of who accepts their offer of admission and how your profile may or may not round out the rest of the class. This is something largely out of your control.
In short, it's your call, but I'd advise hanging tight and hoping for the best.