OK, it's a little obscure, but the phrase "lavishly tooled in naff brown plastic" refers to the musings of Adams in his '80s detective/fantasy novel The Long Dark Tea-Time of the Soul. He posits that Norse deity Thor, were he to materialise in the present day sans birth certificate, could not become a fully functioning member of modern society (but concludes that perhaps this is no tragedy).
D.A. writes: "If, to sustain for a moment the same arbitrary hypothesis, the God Thor were alive and for some reason at large in England, then he would probably be the only person in the country who did not receive the constant barrage of invitations to apply for an American Express card, crude threats by the same post to take their American Express cards away, and gift catalogues full of sumptuously unpleasant things, lavishly tooled in naff brown plastic."
Thor actually features quite heavily in the book... It's probably sacrilege to say so here, but I prefer the Gently series to HHGTTG.