loan out corporations are often established by entertainers, where the performers contract with the media (film, tv, music) to provide the service, payable to the corporation. There are issues of coordinating benefit limits with the union plans sponsored by the guilds where the performer is a member. There are extensive discussions about these issues here at benefitslink.
Otherwise, they are simply corporate plans owned by the sole employee.
Simple? Not that I'm aware of. Once you satisfy the 1 year of service you are in and going to "part time" won't exclude you.
I think what you have to do is come up with a reasonable business classification for the the employees you want to exclude; exclude them from participation in the plan document; test for coverage. Presumably the coverage test wouldn't be a problem unless you have "a lot" of these types of employees.
I have many brokers who 'check things out' with other practitioners who clearly get it wrong. This goes to the definition of "Alternative Defined Contribution" plan. It's somewhere within the withdrawal restrictions regulations for Section 401(k)(2).
Edited: Looked it up: 1.401(k)-1(d)(4)(i) [[This references the section making distribution upon Plan Termination]] and suggests 12 month period from participating in an Alternative DC plan.
Good Luck!