CoachW... a few things...
First, check your scholastic regulations. Our fed in California does not allow participation in any contest while in season. Many camps and showcases include games (contests). This would be a violation of fed rules and subject both the athlete and HS team to consequences.
Beyond that, you should set a culture of communication within your program whether it be regarding camps, showcases, outside lessons, outside conditioning, practices for other sports, conditioning for other sports, etc. The players need to understand that without communication, it is impossible for you to monitor and protect their arms, general health, eligibility and possible consequences to their team and teammates.
Gov, I appreciate the spirit of your post but, unfortunately, when allowed, it usually becomes the better pitchers that are most sought after to play that extra ball and so their arm health is put in jeopardy. During the season, players are already playing/practicing for roughly 15-18 hours a week. There is plenty of time the other nine months of the year for them to play more if they want. HS summer or fall is when a coach can get creative and mix and match the players in the program to various playing opportunities, including the ones he may provide.
Appreciate where you're coming from... My thoughts were keeping it to a simple showcase, where even if you're a pitcher, your counts are limited. But candidly, my two boys are primarily position players where this wouldn't have too much of an effect. But I understand the arm concern by the players current team coach.