Hi Laurie,
This may seem like a strange suggestion, but why not just create your own field-promotedProperty-column for monitoring checkin/checkout status?
Just add a rule that on form load that checks the field. If it's blank, set it to the current person, submit the form, and leave it open. If it's not blank, check to see if it's the same as the current user (you could use username() for this) and if not, switch to a read only view and display some message that so-and-so has it checked out. Then, you can also provide an admin override. :)
Simply solution and you don't have to hassle with the checkin/checkout stuff in SharePoint.