Case Book 7.2.5 D provides good information on player eligibility during Try's and 4th down plays.
7.2.5 D ... With fourth and 6 from the K20, Team K lines up with A21 as the left end; A34, A66, A25, A64 and A86 in the traditional five tackle-guard-center-guard-tackle spots; and A11 on the right end. All are on the line of scrimmage. Players A10, A20 and A5 are in the backfield with A79 lined up in the deep, position as a potential kicker. A25 places his hand on the ball. Prior to the snap,
(a) A20 shifts to the line of scrimmage on the right of A86 and left of A11 and sets for one second before the ball is snapped; or
(b) A11 steps back off of the line and A20 shifts to replace him as the end, where both are set for a second before the ball is snapped. Who are the eligible receivers prior to the ball being touched by B?
RULING: Once A25 placed his hands on the ball, all players in between the ends (A34, A66, A25, A64 and A86) become ineligible and remain ineligible throughout the down. In (a), once A20 assumed a position on the line of scrimmage, A20 became ineligible as he was covered up by A11. A79 is ineligible by number. Only A21, A11, A10 and A5 are eligible on the play. In (b), because A11 stepped back off of the line first, A20 becomes eligible as he would be the end. A21, A20, A11, A10 and A5 are eligible.
COMMENT: When A is in scrimmage-kick formation, it does not have to kick and may kick when it is not in this formation. [2-14-2, 2-32-9, 2-39, 7-2-5b(2), 7-5-6]