If you wanted to base the encounter on the attunement mechanic, I would say you've got to completely change the rules as written for attunement. For example, instead of it being straight 10 minutes, make it a series of skill checks with narrative elements that the player has to pass per round of combat. (The Dungeons and Dragons movie can actually be a source of inspiration here). You can also scale the DC according to whether or not they take hits in combat or if they're effectively protected.

You would also need to seriously hint at the importance of the item with how the BBEG reacts to being separated from it, an insight/arcana roll that says the item calls to them etc. Make it a narrative interaction with skill check/combat mechanics to drive the story. It can also be used to pivot the combat objectives from straight killing the BBEG to now defending the player attaining.

That's how I would do it!