I did something similar in February when I was supposed to fly LAS-ATL-PBI, but found myself in SLC on the day of the flight. The agent at the Medallion counter in SLC was extremely helpful though and gave me boarding passes all the way through after I bought a walkup Y fare SLC-LAS that would allow me to connect. I asked about the possibility of taking the SLC-ATL nonstop instead, but she said that it was not possible because of the seperate tickets.