Hey, it's pretty easy to do.

In the custom format, just type:

"MCY ID "#;"MCY ID "#;"MCY ID 00000";"MCY ID "@


Numbers are taken care of by the first 3 parts, and alphanumeric entries are done by the last (whatever text is entered in the cell will be placed where the "@" is). This doesn't make sure that only 5 characters are entered, but it's simple and easy to create.