If an applicant knowingly withholds prior art information from an IDS, are there ways the patent can be invalidated after grant (other than through litigation)?
The way some of or all claims can be invalidated outside of a patent infringement case in a real (Article 3) court is via an IPR before the USPTO. The grounds in an IPR are limited to the issues of novelty and obviousness as shown by written published documents.
Even in litigation the standards for inequitable conduct were raised considerably a few years ago. It now requires a "but for" situation. That means an analysis that but for the excluded prior art, the patent would not have been granted. And it requires a second prong - showing that it was done with an intent to deceive. Before that a patent could be torn up - in litigation - for an "innocent" omission of an item of prior art that would not have affected the case anyway.
Note, if the ommited prior art is killer enough to be "but for" art, then the excluded prior art (if a written published document) might be used in a successful IPR.