We would enforce this number restriction mentioned in the original post. It's not fair for a team to believe #55 is ineligible, not cover him and then have him catch a pass. However, if the local rules are number restrictions are ignored, then so sorry for him.
Here, there are some things we let slip at sub-varsity levels. Today we had #98 and #99 as the offensive tackles in a middle school game. They were brothers I think and both "obviously linemen"... as I am obviously an umpire... if you know what I mean.
Their team was a little short of players and we just told both coaches that these guys would be considered linemen and ineligible for the game and thus did not "officially" have five on the line 50-79.