If you have a different boost handset (non ptt/chirp) then you can do it online. Log into your account. Look in the upper right hand corner. You'd want to "Update handset". Click on it and put in the number of your different handset. Understand that they're going to turn off service to your Prevail and turn on the other handset. On the last page you'll get the other phone's MSL along with reprogramming instructions. You might want to write some stuff down if you want.
Once that's done, log out, log back in and update your handset again. This time putting back the information for the Prevail. When you're done doing it you'll get the instructions again to program your phone. You wont actually do it. Since the phone already had your information in it. Just turn the Prevail on and the phone will be back on... only thing is that when on the last page you'll get the MSL, phone number and the MIN number.
There you go. You got the MSL without calling anyone.
One thing though. If you're on the $55 a month plan changing the handset to a "normal" $50 a month phone or a $60 a month blackberry will change your billing cycle. Once you get back to activating the Prevail you'll have to pay the monthly bill again to start up service. You might also loose the current month's unused portion. There is a way to get credit for your unused days... but that'll mean calling Boost customer support. If the handset you're activating has been used before, and it probably has, then they'll charge you a $10 activation fee. What? Pay money for stuff!?!?
The way to do it is to do it online this way they do not charge you the activation fee but you do not get credit for the unused portion of the month.
Best bet would be to swap $55 a month phones for the same. I'm on the $50 a month plan so I can swap it for any phone (other than Blackberrys and "updated" android handsets).
Good luck.
PS: If all you're going to be doing is changing the phone back and forth for a few minutes you do not really have to have the phone in your possession. Just find a serial somewhere. Swap the phones back and forth and get your MSL. I've read about some serials floating about in Howard Forums. I'll try to get hold of some old Boost phone and put up the serial here for anyone to use as they need it... but no promises.