the thing to do would be for some enterprising individual to arrange the manufacture of covers, then sell them to all the pickup companies! oops, just let slip my golden idea...
i think the reason noone's ever bothered is that most 7 string players are metal players, and don't want the traditional nickel or gold covers. as far as i know bareknuckle are the only company that do real funky covers, and they don't sell enough 7 strings to justify manufacturing covers. even if they did, only a certain percentage would be ordered with covers.
edit:(does that paragraph get an award for most instances of the word "cover" in two sentences?)