Yes, I've made a trait that spawns a faithful dog. However, I CANNOT get it to spawn someone as a champion character (which was my origional purpose). Here is the code for my dog (a custom unit).

            <GameModifier InternalName="SummonUnit">
                <ModType>Map</ModType>
                <Attribute>SummonUnit</Attribute>
                <UnitClass>CompanionDog</UnitClass>
                <StrVal>Doggy</StrVal>
            </GameModifier>

The problem appears to be with SummonUnit, we probably need to be using something else and I haven't discovered that code yet.

I'd love it if someone could figure out how to spawn a champion. We could do a lot with that code! Could implement taverns that slowly spawn new champions, could implement an "Engaged" trait where you begin with a spouse, could implement a "Loyal Friend" trait where you spawn with a companion, and you could use spells to "summon" new champions.

This summons an npc that counts as a champion (could cast imbue champion on them), but could not marry them (female sovereign, with male champion). Still a work in progress, since it only costs 1 mana.

<SpellDef InternalName="SummonJoffrey">

<DisplayName>Summon Joffrey</DisplayName>

<Description>Summon a Joffrey from a pocket universe to serve with you.</Description>

<Image>FireCrystal_Medallion.png</Image>

<IconFG>Pariden'sReturn.png</IconFG>

<IconColor>88,78,32</IconColor>

<SoundFX>Spell_DarkEnchan_0t2</SoundFX>

<ManaCost>1</ManaCost>

<Range>1</Range>

<SpellLevel>1</SpellLevel>

<SpellType>Strategic</SpellType>

<SpellClass>Defensive</SpellClass>

<SpellTargetType>Self</SpellTargetType>

<SpellDefEffect>

<EffectName>SummonLife</EffectName>

<LocalPosition>0,0,0</LocalPosition>

<EffectScale>0.3</EffectScale>

<EffectDelay>0.0</EffectDelay>

<SnapToTerrain>1</SnapToTerrain>

</SpellDefEffect>

<GameModifier InternalName="SummonUnit">

<ModType>Unit</ModType>

<Attribute>UnitJoinArmy</Attribute>

<UnitClass>Champion</UnitClass>

<StrVal>Joffrey</StrVal>

</GameModifier>

</SpellDef>

Let me know if you find out more and I will let you know if I find out more 

Edit: Found out how to create a spouse for you as well!!  As you can see it was meant to allow you to marry the summoned champions (internal name), but it works good!

<SpellDef InternalName="MarryChampion">

<DisplayName>Summon Spouse</DisplayName>

<Description>Summon a spouse.</Description>

<Image>FireCrystal_Medallion.png</Image>

<IconFG>MarriageSpell.png</IconFG>

<IconColor>88,78,32</IconColor>

<SoundFX>Spell_DarkEnchan_0t2</SoundFX>

<ManaCost>0</ManaCost>

<Range>1</Range>

<SpellLevel>1</SpellLevel>

<SpellType>Strategic</SpellType>

<SpellClass>Defensive</SpellClass>

<SpellTargetType>Self</SpellTargetType>

<SpellDefEffect>

<EffectName>SummonLife</EffectName>

<LocalPosition>0,0,0</LocalPosition>

<EffectScale>0.3</EffectScale>

<EffectDelay>0.0</EffectDelay>

<SnapToTerrain>1</SnapToTerrain>

</SpellDefEffect>

<GameModifier InternalName="MarriageProposal">

<ModType>Player</ModType>

<Attribute>MarriageProposal</Attribute>

<AffectedUnits>Male</AffectedUnits>

<StrVal>Would you like a Wife?</StrVal>

</GameModifier>

<GameModifier InternalName="MarriageProposal">

<ModType>Player</ModType>

<Attribute>MarriageProposal</Attribute>

<AffectedUnits>Female</AffectedUnits>

<StrVal>Would you like a Husband?</StrVal>

</GameModifier>

</SpellDef>

For this spell you will need this, save it to [Your Elemental Documents]\Units\Icons\:

Unless someone can think of a better way, the best way I can think of to be able to get a champion from a pub/inn would be make a set of spells that maybe only work once, that their target is the pub/inn, and it summons them for a cost of gold or something.

Well for me it was the duration line that was screwing it up for me. Once I got rid of that I was able to summon the hero.

Also anyone know if there is some way to make a limit on the number of leaders you can summon? Perhaps making it use an enchantment slot, but anyone know how many enchantment slots you have?

Unfortunately no its not a single cast spell and there is no way to make it so, that I know of. I personally believe that the -1 duration makes the summon an enchantment which would make it single cast, but the spell wont work with the -1 duration.