OLCI doesn't give you the option of standby - it's pretty basic and just allows you to change your seat and pick the number of bags you're checking. I'm not sure which airport they can do it at, but I'd inquire when you get to LAS, and see if they can put you on the standby list from there. If not, just go up to the gate when you get into BOS and ask there. BTW, they might charge you a $25 fee, depending on if the flight you want has seats available.

Also, the 800 number is available 24/7, so you could always call at night sometime.