Is there a way to make a workflow check for conflicting calendar entries? Is it possible to do a comparison of a date+time submitted from a form, against a date+time contained in a variable or LDAP value?

What I'm trying to do in the first case is create a calendar for reserving a conference room, and have a workflow check to see if one user is trying to reserve it during a time period when another user already has it reserved.

In the second case, I would like to have a calendar for reserving company vehicles, with a workflow that checks the requested dates against the employee's drivers license expiration date stored in eDirectory.

Any ideas will be greatly appreciated.