If you are purposefully ruining mana items for mages, what about buffing clarity, Rito Gemz?

Restores 40% maximum mana to you and nearby allies. Your next ability deals 10% of your total mana as magic damage to first target hit.

If heal can have secondary effect Clarity could too. Damage doesn't go into crazy values and probably could even be 15%.

or

For next 5 seconds your all of your mana ability costs are halved/reduced by x%. 160s CD

This version of clarity is best described as 'still be able to cast your combo in a pinch'. You are getting low on mana in lane because enemy is playing very aggresively, but you need to help your team contest a dragon, maybe you got ganked just before recalling, or you desperately need to clear this particular minion wave? This is for you. But if you want to just keep farming and stalling it won't help. Notice how it doesn't allow for infinite sieges, passive mana gain or anything nasty. Possible abuse case on only, but with summoner spell cooldown probably acceptable. ARAM clarity users might get sad with that change.