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I've mentioned the Advocate Cycle from Judgment and Phelddagriff/ Questing Phelddagriff before.

Also Forbidden Orchard.

Anything that gives your opponents tokens is good but the best politically are ones that do it over time rather than all at once.

Varchild's War-Riders are maybe the best example of this.

an opponent's Fact or Fiction can be an opportunity for deal making. You can offer to split it for them and give them a pile of 5 and a pile of 0. You better be getting something you really need in return, though, because that's a pretty impressive advantage.

Bazaar Trader is a Zedruu analog.

Yeah, it's a pretty strong card for its cost even without the political element.

The only trouble is, it's not legendary so it can't be a general. You have to go with regular flying purple hippo for that.

EDIT: Ursapine fits well with what you're going for with the Guildmages. If you have the mana you can basically decide who wins nearly any combat on the table.

What I meant by "what you're going for with the Guildmages" wasn't combat pump as much as cards that are not specifically built to help opponents (like all the cards I'd suggested previously were) but which you can choose to help an opponent if you wish.

That category is a much larger one than cards with effects that specifically help your opponents and I think naming every card in the category would give us a very large list indeed.

For example, any permanent with an activated ability that destroys or damages or shrinks creatures that costs only mana or tapping and not other resources can be easily used to bargain with people when they get targeted by another opponent's attack.

There's dozens of cards with that sort of effect: Plague Bearer, Guul Draz Assassin, Kumano Master Yamabushi, Tangarth, Drana, Kalitas, Kiku, Lightning Crafter, just off the top of my head.

Fertilid lets you make an opponent search for a land and put it on the battlefield.

Forgotten Ancient generates +1/+1 counters which he can then move onto any creature.

These are just the decently powerful ones that I have played with or would potentially play with. There's also slews of less powerful ones like every Tim (Prodigal Sorcerer) effect ever printed.

Nin, the Pain Artist and Acorn Catapult are two cards that sit on the border between the two categories.

Malignant Growth
What about that ones usefulness?

I've found Spectral Searchlight to have become a nice political card in lieu of the phasing out of Mana burn. Float somebody the mana they need to boardwipe to your mutual advantage? Always nice. Float somebody a mana so they use that bomb to wipe out not-you? Even better.