EDIT: So I have been testing my Diplomat and Master Diplomat units out with the <CityBonuses> tags. For example, I have made a special promotion line called "Advocate I", "Advocate II", "Advocate III" which are supposed to insert various levels of culture into an enemy city while at the same time making someone happy there (gives them a reason to tolerate you spreading your nasty culture

.). The tags below are for the "Advocate III" promotion:
Code:
<CityBonuses>
<CityBonus>
<bApplyRival>1</bApplyRival>
<fInfectCulture>5</fInfectCulture>
<fHappy>1</fHappy>
</CityBonus>
</CityBonuses>
However, when I plant my unit in a "rival" city (which I assume means a) not on my team and b) not at war with, also have no agreements of any sort) none of my culture is injected into the city. The happy faces do appear in the city list as "Nearby Units Amuse Us." I know this because I've also given the diplomat units the <bCitySpy>1</bCitySpy> tag and so can see inside their city.
Also, I have created another line of promotions called "Emissary I", "Emissary II", and "Emissary III." These represent parking your unit in a rival unit's city to improve diplomatic relations with them. Example:
Code:
<PromotionInfo> <!-- Diplomat - Emissary 1 -->
<Type>PROMOTION_EMISSARY1</Type>
<Description>TXT_KEY_PROMOTION_EMISSARY1</Description>
<PrereqUnits>
<PrereqUnit>UNIT_DIPLOMAT</PrereqUnit>
<PrereqUnit>UNIT_SENIOR_DIPLOMAT</PrereqUnit>
</PrereqUnits>
<CityBonuses>
<CityBonus>
<bApplyRival>1</bApplyRival>
<fDiplo>.1</fDiplo>
<iBonusRange>1</iBonusRange>
<fDecayRate>.5</fDecayRate>
</CityBonus>
</CityBonuses>
<UnitCombats>
<UnitCombat>
<UnitCombatType>UNITCOMBAT_COMMANDER</UnitCombatType>
<bUnitCombat>1</bUnitCombat>
</UnitCombat>
</UnitCombats>
<PromotionNextLevel>PROMOTION_EMISSARY2</PromotionNextLevel>
<Button>Art/Interface/Buttons/Promotions/ArcaneMentor1Comm.dds</Button>
</PromotionInfo>
I would think that the stacking bonuses for each level of Emissary would appear summed. E.g. three levels of emissary together would add .85/turn of diplomatic relations with an enemy (yes, I know, after testing this does seem a bit excessive). However, they don't appear as stacked; each appears as a separate bullet point under the unit. See screenshot for example. This is more of a cosmetic, bug, and the same thing happens with adding the cultural bonuses together.
Is it intended that my unit diplomacy bonus be capped at +3? I ask because no matter how long my diplomat hangs out in an enemy city, the relationship bonus never goes higher than that.
END EDIT
Ah, ok, well just asking

. Maybe I will just create a "Master Diplomat" unit that allows this ability and is an upgrade from the Diplomat as a workaround.
Would be nice for a future update to have this available for Promotions. Also, to have Diplovotes be available as real numbers, like XP, instead of integers. Imagine, more advanced buildings that grant a Fraction of a diplomatic vote! Pile 10 of them up and you get a vote. Or perhaps leave Diplovotes as integers, but grant each civ in a council 100 of them, Chalid grants 100, Teutorix grants 100, and then other units and buildings can grant 5 or 10 or somesuch.