This idea came up in the Housing forum as part of a discussion of customer banners in general, but seems like it would be particularly simple to implement.
When the guildmaster decides the look of the guild tabard, include an option to apply the design to a guild banner as well. The banner would be available in the guild store as a furnishing item, in the same way that the tabard is bought.
Having several styles of banner - like the ones for the alliances offer - would be gravy. I'd settle for just the medium one. That would permit the chosen dyes, borders, crest, and end shape to display properly, which doesn't always work with the others.