I like the idea of focusing the bottleneck on shards of potential. I don't like the idea of it being super expensive to make them, though, since that serves no purpose other than widening the gap between the haves and the have nots.
What about a shard timer, like a raid timer? Isn't that already a tried and true method of limiting raid loot? How about any shard of potential that raises an item's level above the current level cap, you can only make one of them every x days, x being the difference between it's expected ML and the level cap.
Specifically, a +11 shard (ML21) would be on a 1 day timer, +12 (ML23) on a 3 day timer, etc... This would have the added bonus of being a self-adjusting system if you ever raise the level cap.
EDIT: I imagine all three "flavors" would be on a single timer, but hey, that's not even necessarily a requirement. It would still stop an uber-crafter from flooding the market if they had three timers per level.