The main tag for UNIT_SCOUT Type would be CLASS_RECON. That's mainly used to assign promotion types and to allow various units such as Great Generals to apply their special abilities to them. Are you looking to allow specific other units to embark earlier or are you trying to get recon units themselves able to do so?
There are a few basic manners in which this could be accomplished but how you go about it would depend on your specific goals, I'd say. If you want every civilization to have that option, that'd be a very different solution than if you want to grant a particular civilization the option.
For the latter, repurposing part of Maori's traits would likely be one of the more straightforward methods. The TRAIT_MAORI_EMBARKED_ABILITY modifier in that trait could be applied to the class for whatever you're looking at doing this for.
You can find the following code for that specific part of that trait in the file Expansion2_Civilizations_Major.xml located in <install path>\DLC\Expansion2\Data :
XML:
<GameInfo>
<Types>
<Row Type="TRAIT_CIVILIZATION_MAORI_MANA" Kind="KIND_TRAIT"/>
</Types>
<CivilizationTraits>
<Row CivilizationType="CIVILIZATION_MAORI" TraitType="TRAIT_CIVILIZATION_MAORI_MANA"/>
</CivilizationTraits>
<TraitModifiers>
<Row>
<TraitType>TRAIT_CIVILIZATION_MAORI_MANA</TraitType>
<ModifierId>TRAIT_MAORI_EMBARKED_ABILITY</ModifierId>
</Row>
</TraitModifiers>
<Modifiers>
<Row>
<ModifierId>TRAIT_MAORI_EMBARKED_ABILITY</ModifierId>
<ModifierType>MODIFIER_PLAYER_UNITS_GRANT_ABILITY</ModifierType>
<Permanent>true</Permanent>
</Row>
<Row>
<ModifierId>TRAIT_MAORI_EMBARKED_ABILITY</ModifierId>
<Name>AbilityType</Name>
<Value>ABILITY_MANA</Value>
</Row>
</Modifiers>
</GameInfo>
You could just take that trait modifier and apply it directly to a different civilization's trait like this:
XML:
<TraitModifiers>
<Row>
<TraitType>TRAIT_CIVILIZATION_MALI_GOLD_DESERT</TraitType>
<ModifierId>TRAIT_MAORI_EMBARKED_ABILITY</ModifierId>
</Row>
</TraitModifiers>
I just grabbed Mali's since they're in the same file I had open but you could do that for any civilization. To apply it to units for all civilizations would work differently, of course, but the same modifier could be used like so:
XML:
<GameInfo>
<Types>
<Row Type="ABILITY_EMBARKATION_FOR_LONECAT" Kind="KIND_ABILITY"/>
</Types>
<TypeTags>
<Row Type="ABILITY_EMBARKATION_FOR_LONECAT" Tag="CLASS_RECON"/>
<Row Type="ABILITY_EMBARKATION_FOR_LONECAT" Tag="CLASS_MELEE"/>
</TypeTags>
<UnitAbilities>
<Row UnitAbilityType="ABILITY_EMBARKATION_FOR_LONECAT" Name="LOC_ABILITY_EMBARKATION_FOR_LONECAT_NAME" Description="LOC_ABILITY_EMBARKATION_FOR_LONECAT_DESCRIPTION"/>
</UnitAbilities>
<UnitAbilityModifiers>
<Row>
<UnitAbilityType>ABILITY_EMBARKATION_FOR_LONECAT</UnitAbilityType>
<ModifierId>TRAIT_MAORI_EMBARKED_ABILITY</ModifierId>
</Row>
</UnitAbilityModifiers>
</GameInfo>
This is, of course, untested but the principle is sound and should work fairly well. While I only applied it to both recon and melee units above, you could use a single class, of course, or add as many as you like, one to a line, in the TypeTags. That would then make it available to all civilizations who train units of that class.
Edited to add: You'd need to make a separate file with the descriptions if you used the last section of code. That's under <GameData> and you can't put both of those in a single file. I didn't bother adding them here since this would work and you'd simply see LOC_ABILITY_EMBARKATION_FOR_LONECAT_NAME for the ability name, if you saw it anywhere at all. Unit abilities aren't really in your face or anything so it should be easy to ignore but of course if you want to publish anything it's best to add that as well.
