Same thing has happenned to mine, even though I have original rear mudflaps! Been rusting through from the underside for years and the top surface is all peppered with tiny brown specs of rust! Clearly been like that for some years as it hasn't got any worse since I got the car.
I took the bumper off and rubbed down the underside before giving it a good thick coat of Hammerite to stop the rot going any further. Won't make any difference now, the damage is done, but it's something I suppose.
Adam's right, decent back bumpers (or just straight ones) are a bit like hen's teeth. Wins ltd have a few good used examples for about £95 inc VAT and p&p, but if you want a properly top-end one, it's either NOS (Needle Of the hayStack) or take the hit and do what chris' did and repair and rechrome! :?