Great question and the NCAA site isn't specific in regards to that exact question. They do have this.
quote:
Redshirt Definition
The term "redshirt" is used to describe a student-athlete who does not participate in competition in a sport for an entire academic year. If you do not compete in a sport the entire academic year, you have not used a season of competition. For example, if you are a qualifier, and you attend a four-year college your freshman year, and you practice but do not compete against outside competition, you would still have the next four years to play four seasons of competition.
Each student is allowed no more than four seasons of competition per sport. If you were not a qualifier, you may have fewer seasons of competition available to you. You should know that NCAA rules indicate that any competition, regardless of time, during a season counts as one of your seasons of competition in that sport. It does not matter how long you were involved in a particular competition (for example, one play in a football game, one point in a volleyball match); you will be charged with one season of competition.
The way I read it, the intent is to allow a player who is redshirting to scrimmage with
HIS team only. If it
IS a grey area, I would bet that the coach wouldn't risk loosing a year of eligibility for a player in a meaningless contest.