You'd need a very small hex key to do it in-situ. Did one come with the kit? The ones that tend to come as standard in the kits are normally far too long. You'd need one with a really stubby end to do it. Also depends on how recessed the end of the truss-rod nut is. I'm not saying it can't be done, but unless you can get hold of the right tool, it may be better just to cover it up. Have a look and see what you think.