Rules FAQ

The requirements for being declared for a challenge are as follows (RR, p28): In order to be eligible as an attacker, a character must have a challenge icon that corresponds with the type of challenge being initiated, and the character must be standing.

Jaime's ability does NOT allow him to be declared as an attacker if he's already kneeling. Compare with Obara Sand, who specifically states she can be declared (as a defender) while kneeling.