This will work if you add it within a mod's <* Class="PatchOperationSequence">

Otherwise, the code looks something like:



<!--Cataphract Armor-->

<Operation Class="PatchOperationAdd">
<xpath>/Defs/ThingDef[defName="Apparel_ArmorCataphract"]</xpath>
<value>
<apparel>
<tags>
<li>Spacer</li>
<li>EVA</li>
</tags>
</apparel>
</value>
</Operation>




But it depends on various mod author's way of implementing items, naming, and how they sort stuff within their .xmls. Sometimes <Insulation_Cold> has to be added instead of replaced, or you need an @name structure, since you're modifying 'abstract' armors - a base list of properties, that a specific armor loads first, and maybe doesn't even modify, as is the case in vanilla armors, note the <xpath> section above vs below:




<Operation Class="PatchOperationReplace">
<xpath>/Defs/ThingDef[@Name="ApparelArmorCataphractBase"]/statBases/Insulation_Cold</xpath>
<value>
<Insulation_Cold>100</Insulation_Cold>
</value>
</Operation>




vs.:




<Operation Class="PatchOperationReplace">
<xpath>/Defs/ThingDef[defName="VAE_Headgear_HeavyMarineHelmet"]/statBases/Insulation_Cold</xpath>
<value>
<Insulation_Cold>100</Insulation_Cold>
</value>
</li>
</Operation>

Either way, thanks for the initiative and a great guide. If anyone does their own modifications to a mod, I'll gladly merge your code if you post it here.