Rob,
The NCAA rules allow a college to make a written offer of acceptance and/or financial aid to the college, and it may be signed by the player at the early signing date. In the Division Manuals it is termed simply a "written offer of admission and/or financial aid." Perhaps some conferences (e.g. Ivy or Patriot Leagues) may have a more elegant name for this. Once such an offer has been signed, the college may publicize its recruitment of the player.
However, it isn't clear to me how this guarantees a roster spot, nor what recourse a player would have if a written guarantee were provided but not followed. Note that an NLI also doesn't guarantee a roster spot--it just (in principle but not in practice) guarantees the athletic aid. Any player receiving athletic aid is a counter and in baseball must be included on the 35 man squad list, but that doesn't require the college to allow the player to practice with the team.
CaliBaller,
I agree with the already posted answers to your questions. I'd like to point out that even a coach who is acting in good faith may have reasons to not include the player of the squad list--thus meaning he wouldn't be able to practice with the team. The most obvious is an injury which will keep the player from contributing during the baseball season. Additionally, if the player arrives at fall practice in poor physical condition, e.g out of shape or with a chronic injury which leads to regression in performance, then probably the coach will feel that the player hasn't kept up his end of the bargain.