OK, how about if some monsters had an "encounter power" that was specifically designed for an enemy to use against it. Effectively, the monster donates an encounter power to an enemy, that the enemy (in most cases a PC) gets to use if they use an action point for example but that might have additional pre-requisites. In this way, any PC can "use" this encounter power but they have to meet the pre-requisites (like being in the right position or making a difficult athletics check etc.) to do so, but with spectacular results a la the Beowulf dragon example. Yeah... that could be fun.
I've had similar thoughts about "object-oriented" monster design, in which each monster (object) comes with a set of powers (methods) that PCs could use on the monster, instead of just powers for the monster to use against the PCs. In the end, I decided that this idea had two (related) problems. First, how do you communicate these options to the players without unduly biasing their decisions or "railroading" them? Second, codifying one creative attack against a monster as a power might bias you against other creative attacks that you didn't foresee. In other words, after deciding how to implement hopping on the dragon's back, what do you say when a player says he wants to pin the dragon beneath the portcullis?
IMC, I've decided to take a fast and loose approach to creative tactics. If a player wants to jump onto a dragon's back, great! I'll model it as a grab, but instead of immobilizing the dragon, the character will be dragged along when the dragon moves. The dragon won't be able to make claw attacks against the character, but the bite seems fair game. (Not decided yet about the breath weapon.) The PC will have combat advantage, and the dragon won't be able to make opportunity attacks against any point-blank "ranged" attacks. If they want to drop a portcullis on the dragon as it flies through the castle gate, it'll be a limited damage expression from DMG page 42, perhaps leaving the dragon immobilized (save ends).
In short, for better or for worse, player creativity in battle is probably best handled by a flexible DM, instead of any rules you could possibly codify. As a newbie DM, I've often hoped that well-designed rules could compensate for my inexperience, but the solution turns out to be just to DM more (and earn DM XP), not to create more rules.
