What would be the point of going and communicating with new schools? To let them know you'd love for them to be a backup option? If he doesn't get in he can always apply to other schools, explain the situation and show up the first day of tryouts. If you're good enough they'll keep you, they don't have scholarship guys to account for. Recruiting means the least at the D3 level, your game determines your role, not so much the commitment you made.
That being said, unless travel and all that is already paid for I would just skip it. No sense in reaching out to new schools now. We're 5 weeks out from ED applications being due and realistically they're all done 2 weeks prior. Part of committing to a school is being able to get in academically. If the coach can't guarantee admission then it probably isn't a school he should be committing to.
I am again probably showing my relative ignorance with D3 as I certainly have more direct experience with the other levels. Still, I have had a fair number of players from our program end up going D3. Most of those players still considered baseball to be a very big part of the equation.
Again, the OP says acceptance is far from a sure thing. With recruiting at other levels, coaches generally would be more than happy to take a drop down should things not work out at the player's first choice. OP is a senior. Sure he could wait to see what happens with the ED acceptance process but wouldn't he be in a better spot if he has already had some dialog with other schools where there may be some mutual interest? Wouldn't he potentially be in a better spot if some schools saw him at the event and became interested, and in turn became solid options should he not gain acceptance to school A? I know D3 recruiting often happens late, but by mid-senior year, the later in the game it gets, the more challenging it becomes in many ways... time to generate interest, time to visit, etc. It is definitely worth while, IMO, to avoid putting yourself in a last minute chase or settle scenario.