You CAN service shimano freehubs – I got a tool made at work (just a piece of round bar milled with two notches sticking out of the bottom. Don't forget to unscrew it CLOCKWISE…….. I guess there is a proper shimano tool somewhere.
Everybody also says shimano singlespeed freewheels are useless – every time my work one has gone "scrap" a simple lunchtime strip and service has got it running for another 12 months. The only tools needed are a hammer and nail to undo the outer race!
Adjusting cup / cone hubs is much easier if you hold the drive side axle locknut in a vice (axle pointing up). This lets you do minor tweaks of the non drive side cone and locknut without the axle rotating.
Mrs had some XT hubs circa 1996 that didn't have today's big outer rubber seal. Instead they had a smaller rubber lip seal on the cone and a greaseport in the dust cap (rotate the dust cap to open / close the port – worked great).