OK, for what it is worth, I finally found something concerning safety of wood vs metal bats. I found it on a site called "don't take my bat away" which is apparently is a coalition put together to combat the banning of metal bats. So far, other than a 2002 study, this is all I could find. Incidentally, the study was conducted on behalf of the Illinois High School Association and funded by the National Federation of State High Schools Foundation.
First On-Field Study Comparing Wood to Non-Wood Bats Finds Both Safe NFHS Comments On IL Field StudyThe research project was commissioned by the Illinois High School Association (IHSA), with research
conducted by the School of Kinesiology and Recreation at Illinois State University. Funding was provided by the NFHS Foundation.
“Based on the results of this study, we have determined that using nonwood bats results in a greater number of hits per game and a longer duration of games when compared to wood bats among high school baseball players. However, there was no statistically significant evidence that nonwood bats result in an increased incidence or severity of injury,” said Kevin Laudner, assistant professor in the School of Kinesiology and Recreation at Illinois State and principal investigator for the Illinois bat study.
Thirtytwo IHSA schools submitted data on wood bats from spring 2007 baseball games, and 11 of the 32 reported data for games played both with wood and nonwood bats.
In the 412 games played by the 32 teams using wood bats, there were 368 broken bats, resulting in a batbreakage
rate of 28.3 per 1,000 atbats.
If the bats were just used in games, this rate calculates to 23.49 broken
bats per team for the entire season. Stated another way, a wood bat could be used for an average of 35.5 atbats
before breakage.
More...
The participating schools reported five injuries in games with nonwood bats in a total of 4,682 atbats, compared with two injuries in games with wood bats in a total of 4,462 atbats; however, ISU researchers said, after analysis, these data show that there is no statistical difference in injury rates when using a nonwood bat compared to a wood bat.
Of the seven total injuries, only two caused the players to miss playing time, and neither of those involved a
bat. One player was hit in the face by a pitched ball and another sustained a blister on a finger. None of the injuries
from a batted ball required any player to lose playing time – in either the woodbat
games or the nonwoodbat games.
The idea for the woodbat study came from the Illinois High School Association.