My interpretation (RAI) is that when they say "You can't use a ranged weapon as a melee weapon", they mean to say (for example) "You cannot fire arrows out of a bow and call it a melee weapon" and thus I would allow a bow to be used as an improvised weapon.
This, definitely.
The rule is there to prevent players to abuse class powers or claim they're not getting an OA because they're firing their arrows to an opponent in melee.
If you use a bow to whack people around, you're not going to get any mechanical benefit (actually, since it does the same damage of other improvised weapons, unarmed attacks included, all you're going to get is some raised brows), I see no reason to prohibit it.