Hi Don
How do you seal the masonite panels before attaching the print to it?
My main use for masonite is for artwork done with oils directly on the panel or sanded pastel paper dry mounted to the panel. I seal it with gesso for the paintings which is not what you would want to use, but any archival sealant would work. A self leveling synthetic clear or colored sealer sprayed on would give you a flatter surface to dry mount to than brushed on gesso. You can sand out and re-seal any blobs that might happen in the sealing process. I have quite a bit of free time, being semi-retired, so the time it takes to prep a sheet of masonite is well worth the savings for me. I also prefer to paint on the panels that I prepared myself vs. commercially made stuff.
You will want to seal both sides and the edges to prevent warping and to make it an archival support. It is pretty difficult to cut and finish perfectly compared to foam core or mat board so unless you have a table saw and other tools you may want to have the lumber yard cut to size first for you. Without the right tools and some time spent, a mounted print with exposed edges and no frame on it might be difficult to make it look nice, but if you are putting them in frames, then the look of the edge is not that critical. If you added a furring strip frame (like a stretched canvas has) to the back of it, you could do "gallery wraps" that would be quite durable and flat but with some serious protection and support for the canvas in the middle of the frame, which a gallery wrap does not have.
I have not mounted any photos on masonite personally, but based on how well it holds up with my paintings on it, I would say it is worth a try considering how cheap it is. Even the 1/8th inch stuff is definitely more rigid than mat board and more durable than regular foam core ( and a LOT cheaper than either of those too). Be sure to buy it from a lumberyard that keeps it stored flat and indoors. The Home Depot here has it on three arm supports and their entire inventory is warped and wavy because they are not properly supported, though they do eventually lay flat once cut to size unless they have been seriously mistreated by the store.
You can also buy hardboard panels dimensionally cut and ready to prime from some art supply sources. Jerry's has the Ampersand brand in various styles. I don't know how the pre-made panels compare in price to the Mightboard and Gatorboard but if you don't want to deal with cutting them yourself, check them out.
http://www.jerrysartarama.com/discount-art-supplies/canvas-and-boards/canvas-panels-and-boards/ampersand-hardbord-panels/ampersand-hardbord-1/8-flat-panels.htmFor what its worth, Bay Photo offers masonite dry mounting:
http://www.bayphoto.com/mounting-finishing/masonite/Several other photo labs offer masonite mounting of their prints.