I don't know anything about letters of interest but it's completely fine to ask how things look for you if your'e on the HPWL. However, you should probably wait like another 10-14 days because they are still finalizing since it's not 5/1 yet. Last year I actually called just to tell them I was taking my stats class and they asked me if I wanted to hear if I had a good shot or not, so they are definitely open to talking about it.
Also I was given an offer on 5/15 last year so they definitely start moving that list really quick!