It is doable, but a major pain. My colleague flashed a DROID RAZR to BOOST, but it required going through SPRINT Corporate (IIRC he dealt with somebody in Mr. Elfman's office) and working with a few people to get the devices MEID added to SPRINT's/BOOSTs database and installing a BOOST PRL.
At first SPRINT tried to sell my colleague on a BOOST phone, but he pressed hard enough and eventually SPRINT Corp. did help him out. They indicated (according to my colleague) that they did not want to make a habit of adding "foreign phones to their database, especially BOOST", but that they would evaluate each request on a case by case basis
It probably helped that my employer uses SPRINT as a vendor and we give SPRINT a lot of business and the initial contacts were setup by our SPRINT Corp. Sales Rep.
Dan