As for hiding packs, i've done one show where the male lead ended up in just a pair of y fronts. The pack was placed down the front of them and he was wearing a very tight dancing jock strap so the natural "bulge" you'd expect was actually the beltpack. Needless to say i let him fit the beltpack!
The stage show of "The full monty" also poses problems for obvious reasons. The 5 male leads do end up completely naked. I've seen this done 2 ways, either backing tracked for the last few lines, or with beltpack and mic hidden in the hat they wear right until the final reveal. The last line is when the hat then becomes unuseable as it's moved down to preserve their dignity which then needs some careful thought, but luckily the last line is delivered pretty loud so the boundary mics can do the job.
But yes, it is very much an artform, it takes an awful lot of time pre-show to get everybody rigged, especially when it's a 20+ mic show and you've no A2!