I do! It's pretty easy and can be done with the *.ext format. For example, make a OrkBoyz.xml.ext file in your ./mods/YOURMOD/Data/World/Units/Orks/ directory with this code:
<unit>
<strategyModifiers>
<modifier>
<effects>
<increaseInfantryScore base="XXX"/>
</effects>
</modifier>
</strategyModifiers>
</unit>
Where XXX is decimal number representing the strength of the recruitment, the default being "1.0".
If you want to have the Ai not research or build a unit, set it to "-1".
There are a few types of unit strategyModifiers, Ai will try and have a mix of ...
Infantry
Aircraft
Heroes
Vehicles
Colonizers
If you want to turn off all Heroes, for example, you have to set all the increaseHeroesScore base to "-1". I find if a unit has a base="2", the Ai will prioritize it over other units until it has many, many of the same type.
There's a productionWeight, too, in the header, like <unit productionWeight="1.0">, invisible in most units because the default is one. This is strength of, "OK build some but not too many."
So together, if you want just a single unit of a type, give it a high strategyModifier but a productionWeight of 0.01.
Hope that helps!

Thanks, this sounds spot on. I'll give it a go and see how it turns out.