Hey Audiofox,
Issues for re-veneering are plenty, for instance glue compatability to substrate, type of veneer backing (phenolic, paper, foil), type of finish to be applied to veneer (lacquer, shellac, varnish, etc.) When buying your veneer you must be aware of it's backing otherwise depending on finish to be applied you'll get bubbles if you run into compatability issues, you also run the risk of loosening the existing veneer if you use too much lacquer thinner or too much contact adhesive, that would be your worst case scenario for at that juncture you will have to strip down your speaker to the MDF and start from scratch.