Attention, all hands! Col. Lacy's surgery has been declared justified by the following tropes:
Artistic License Military: Unless Hawkeye was able to perform the operation completely on his own, there's no way that Potter would not have found out about it. And it would have been a court-martial offense. (On the other hand, given that Potter was already drafting a report to higher echelons that bluntly stated that Lacy was not fit for command, he may well have been willing to cut Hawkeye a tiny bit of slack this once...)
Colonel Kilgore: Lieutenant Colonel Lacy fits this trope, as he has an almost single-minded obsession with taking a certain Communist-held hill. Not for any strategic or tactical reason (and it's pretty much spelled out that taking that hill won't shorten the war by a millisecond), but because it'll satisfy his own sense of martial glory and put him in good stead for a promotion.
Distinction Without a Difference: Even after being deliberately ordered by his commanding general to stop assaulting that hill due to the high cost in lives, Lacy says that he will still attack the hill, but under the cover of a reconnaissance probe gone bad. He says this to Hawkeye, which spurs Hawkeye's plan to cut out his appendix to keep off the lines for as long as possible.
First by Hawkeye, when he finally realizes he may well have violated his medical principles and for nothing (yes, he took Col. Lacy off the playing board for a while, but the wounded kept coming in, all the same). You can see the utterly defeated look in his motions as he gets up reluctantly to go back to the OR.
Secondly by Klinger, when his fake voodoo curse apparently strikes the wrong target. He was so remorseful over what (he believed) he'd done that he immediately gave over all his Voodoo gear to Fr. Mulcahy for disposal (except for the chicken, which he decided to return tothe kitchen...)
The Neidermeyer: What Lacy's men actually think of him. One wounded soldier comes as close as CBS Standards and Practices of the time would allow to telling Lacy to shove the Purple Heart he was about to award him up his ass, while another goes into cardiac arrest at the mere sight of him.
Real Life Writes the Plot: The argument between Hawkeye and B.J. about cutting needlessly into Col. Lacy was not in the original script as written by Tom Reeder. In the original script, both Hawk and Beej were equally ready to take out a healthy appendix, just as Hawkeye and Trapper did to Col. Flagg in the early-series episode "White Gold." Mike Farrell strenuously objected to this, considering it ghoulish to cut into a healthy patient for any reason, while Alan Alda argued whatever was necessary to get a Blood Knight off of active duty was justified in the name of saving lives. Their Real Life argument fascinated Reeder, who revised his script to include Farrell's and Alda's real-life viewpoints, and ultimately saw Hawkeye perform the surgery alone when B.J. refused to participate. The end of the episode ties in as well, as Alda and Farrell's falling out had apparently strained their friendship and the characters reconciling was indicitive of the actors doing so as well.