Since you aren't interested in making a version of MrPoster that will run native under Linux, perhaps you'd be interested in making the regular Windows version compatable with the Wine API for running Windows apps under *nix?

Codeweavers has an "installer challenge" now for helping companies to make their software work with Wine, so that you aren't locking out a chunk of your potential audience. http://www.codeweavers.com/compatibility/challenge/