i dont think its tacky, especially if you are trying to convey a specific theme idea for the wedding as a whole.
for example, i had a huge at home wedding before our cabo wedding. we didnt do the boarding pass idea, but we did do the message in a bottle STD's, and we had palm trees at the "altar" and went totally beachy with the theme, from starfish and sand dollars and sand in the centerpieces, to a grass mat aislerunner, to chocolate seashell cupcake toppers, etc.
i think the more you keep up with the theme idea, the more people will feel like they participated in a "tropical destination" wedding, even if not everyone had to travel far or if you were really in a beach location or not.
basically i say go for it!! remember, boarding passes are for ANY destination, regardless of in or out of the country.