Looking at your picture, what's the concern? It's not like you have specialised custom plates or anything.
Reversible screws available from AutoPro. I got them because I have custom plates. Less than $10, they install with a standard screwdriver and if you keep your receipt you can go back to the shop you bought them from and they have a special attachment that will take off the security screws and allow you to reuse them.
And I wouldnt rivet number plates on. More screws = more damage. I'd rather have my plates remade for $50 then having a panel beater charge me $1000 to repair and respray the entire front guard. Like has already been said, if someone really wants your plates they will take them.
Security screws, number plate covers (maybe even superglue these on if you are really worried), and that should be more than enough to deter most thieves. Actually, there's nothing stopping you gluing the plates onto the bumper with liquid nails and THEN security screwing them in place. That's getting a bit extreme, but I guess it's just another way to lock it all down. How far do you want to go?