During our latest staff training we had the pleasure of a young flexible staff member who showed off his skills when restrained in a face up (supine) position. The staff memberwas able to roll upward onto his upper back and shoulders and deliver knee strikes (with force) to our heads. Granted we would not elect to use this hold for someone like him who has no restrictions for the standard prone position but for demonstration and training we were at a loss for an answer.
Most people do not have the flexibility to present the problem you describe, which is why it normally not necessary to have a leg person. In the scenario you present, you will need a person to control the client’s legs.
For his safety, make sure the person assigned to control the legs 1) stays low on the way in (using one of the people on the shoulders to block the resident’s view) and 2) he aims for the waist and works down to the legs.