Strong magnets should do the job. Make sure they are really strong!! Use double sided to stick a strip of thin metal on your number plate, then glue/masking tape, silicone, etc strong magnets behind the bumper where you want your number plate to be.
Examples: http://www.amazon.co.uk/First4magnets-20mm-Thick-Neodymium-Magnet/dp/B0038E2KYE/ref=pd_bxgy_60_text_y
http://www.ebay.co.uk/sch/Magnets-/147829/i.html
Make sure you have 3 or more individual contact point to avoid it moving about whilst driving at speed. If you are going to place it on a curved surface, make sure the metal plate is shaped to match ensuring best contact before sticking it onto the number plate. Also ensure the surface is clean before attaching the plate as the strong magnet will pull the plate and might scratch your bumper.
Note: I've never done this before but just an idea for you to explore. I've used these magnets to stick 'L' plate on my previous mini and it works really well.