Which champions would you want to buy a viable Manamune?

Depends on the situation. Sometimes you want it because of mana but later end up selling it because there's much better items. Blitz was one of those champs that got it. Pretty much any AD champ who uses a lot of mana.

revert it to doing true damage so I can buy it on Urgot

Essence reaver essentially removed Manamune from the game.

There are no good mana builds anymore since they nerfed frozen gauntlet.

Even Ryze is unplayable nowadays. TLDR Mana builds suck thanks to the utility of other AP items

Always fancied trying it on Nasus. You max Q anyway, plus Q only costing 20 anyway the 15 refund helps in lane with a dorans ring start, might take a little while longer to ramp up but the power spike you could get. Helps if you are against a heavy ad team with iceborn and frozen heart obviously.

... no? Silly idea.

If I had 7 inventory slots instead of 6 I'd put it in a Taric build

Two hour game Cassio