I like the idea, but I think the passive's Cooldown should be based on the imbued ability's CD, probably equal to 1.5x the imbued abilities CD. That way it can only be trigger once per use, it can be put on Ultimates or base abilities without breaking the item, and if someone grabs Transcedent CD they can use it functionally once per ability CD so long as they are willing to wait the extra 20% of the imbued ability's CD for this to come off CD.
I would also indicate that it increases the rate that Channeled abilities are channeled. This is important for things like Holliday's Lasso which technically has a channel time before the lasso is thrown out. Since it's imbued, heroes like Dynamo can pick it up on their stomp and still get value out of their ult (as it would otherwise reduce the duration of his ult, which he wouldn't want) or you might see it on Victor as it could reduce the delay (technically a channel time during which he is untargettable) until his ult goes off.
Actually, yeah there are a lot of heroes who could really use something like this and if the CD is based on the imbued ability it could work really well