No to clippers. But you already knew that. Except maybe poop cute only, directly beneath the tail. May not be appropriate for show dogs????
I don't see a problem with using thinning shears, but undercoat only! I had decide to take off Bacca's butt hair and she took the guard hair to. What I ended up with was unruly undercoat with no guard to keep it flat. It took two years to grow back properly.
But in reality, I've found that a bath and high velocity blow dry, followed by a good brush and rake, thins Indy's pants beautifully. And he has really fat pants, though not so long.
Then youcan neaten the lines with a little scissoring.