I presume you've already read AA's advice on large print special mounts in The Print, in which he suggests finishing by lacquering the surface of the print . Masonite being the backing, I also assume that archival considerations are simply not relevant in your case. Nothing wrong with that, for special applications.
FWIW, I dry mount GSPs that large in a normal size press, presented in a beveled window mat, 36"x48". Since I have not mounted on masonite, I can offer no experiential wisdom. I do have an idea (not to be confused with wisdom) about this though, which might even be considered archival. Here is how I might handle a loose print on any kind of flat non archival substrate. Cut the masonite, plywood or what have you to the size of the print, with or without borders (flush). Go to the home store and buy sufficient length of aluminum C-channel (available in many depths) for each of the four sides, mitered to length, deep enough to accommodate the panel, some kind of archival mounting board or tissue, the print, a window, and acrylic glazing, narrow enough to hold the components tightly (makeup depth as needed). Sandwich them together, apply the C-channel all around with glue on the substrate's back, and hang. The print, only in contact with inert or buffered surfaces, can be removed at any time, by simply pulling the C-channel from the substrate. Then there's the option of incorporating the substrate itself into the presentation by cutting larger than the print, for an aesthetic reveal, not that I would consider masonite all that aesthetic.
Last edited by ROL; 10-08-2013 at 12:18 PM. Click to view previous post history.