Political Wire flags the issue. The federal statute bars making such promises “for the purpose of procuring support in his candidacy.” I read this as requiring proof Gingrich said he would appoint Bolton for the purpose of getting Bolton’s support. More likely, Gingrich made the promise to gain support from voters.
I recall a similar issue around alleged promises of employment in the Obama administration to Joe Sestak to get him to drop out of the race against Arlen Specter. After nosing around, I found virtually no prosecutions or caselaw developing such rules.