You're right, they are extremely similar devices. The primary reason why this isn't possible is due to the locked bootloader - various checks are in place to make sure that the phone will only boot a kernel specifically built for the Milestone by Motorola.
Attempting to flash any Droid rom will result in a device that won't boot - you'll have to flash it with a stock sbf or with a nandroid backup, if your recovery still works.
A small correction, both the Droid and Milestone have identical CPUs. They're both 600MHz Cortex A8's, underclocked to 550MHz.