I have a list defined with a column called "Priority". The Priority column is a choice column where the list of choices is defined as: 1,2,3,4,5 and None. This effectively defines a list item as being one of the top-five priorities or as being unprioritized. 

I would like my users to be able to set the priority for multiple items in the list to "None" (i.e. many list items might be unprioritzed). I would also like to restrict my users from being able to assign multiple items in the list at priority 1,2,3,4 or 5 (i.e. we can't have two items as the top priority).

I've looked at column validation, list validation, workflows and I can't find any way to make this happen. Is it possible in SharePoint Foundation?