Yesterday on the Babylon 5 subreddit, a clip was posted which appears to show Pat Tallman as a background character in The Undiscovered Country. If so, that would be her first known appearance in Star Trek, after which she appeared as another nine* characters in addition to stunt doubling eighteen times for fourteen actresses according to Memory Alpha, which currently doesn't include her apparent appearance in Star Trek VI.
*Her two appearances as an Enterprise-D officer and possibly also as a Defiant weapons officer could be as the same character.
*Her two appearances as an Enterprise-D officer and possibly also as a Defiant weapons officer could be as the same character.