This stuff
http://m.countdown.co.nz/ProductDetails/79794
On my phone atm, but if you go to my project fb page, look in my "other cars" album, there should be a couple of pics and maybe a vid of my sti at a drag racing event with the race number written on this way.
That's all they do here in NZ. Aside from guys with dedicated race cars with permanent numbers, I've only ever seems race numbers written on using this product.