Oh boy, you may be in a good ride on this one soooooo I'll start it off and get my opinion out of the way.
IMO, your are trying to be "nice" about the punishment. If that is your thing, go for it. But, there is no substitute punishment for missing the one thing they want the most...play time...again, IMO there is NO substitute. If you are not willing to sit a player as punishment, because you are short on talent or numbers, then you are not willing to send a strong message. I speak to this from my experiences as a player.. In HS the starting 3B spot was mine to win or loose......I lost it because I was Mr Funny all the time...and lost it rightfully so. Almost the single best thing the HS Coach ever did for me.
You can run a kid, take them out of their comfort zone by fielding when they usually don't, or attach a negative consequence to someone elses activity, but, if in the end he still starts and misses no play time you have accomplished nothing, other than maybe making him a good fielder. IMO, this also sends a negative message to the kids that work hard, do the right things but just seem to be a little less talented. It screams to them that if you have talent you can pretty much do whatever you want, pay a little price, and your still going to get your start/play time