I could definitely see that something around the areas of contact could be useful.
If you're trying to sell to beginners, then some deterioration to the sound won't probably matter (as they probably aren't going to be on the best of instruments anyway). Also if you're on a cheaper instrument, then a cheap little plastic cover or something would be just what's needed to prevent relacquering (which if you get professionally done can be expensive). But if you're selling to people who are keen/pro players, then they may well have spent thousands on an instrument (which may be antique, custom made, or/and adapted to their tastes) and they are quite prepared to pay a significant amount to a specialist brass instrument engineer to aid in the upkeep of the instrument. The sorts of people who have rolls royces in the automobile world, don't want a cheap crappy looking radio in the car - they want style, functionality, and excellence - and it's exactly the same in the instrument world.
If you're aiming for the beginner market, product a product for a beginner. If you're aiming for the pro market, produce something of excellence, like a soft leather cover for where the hand comes into contact with the slide. Get you're hands on some expensive instrument cases, and look at the craftsmanship that went into making the case for the instrument. If an accessory is actually going to make it onto the instrument itself then it really needs to be well crafted, and highly thought through.
