[Joker119]

With more tinkering, I've come to this function for my tag

oUF.Tags.Methods["diablo:name"] = function(unit)
    local color = oUF.Tags.Methods["diablo:color"](unit)
    local name = UnitName(unit)
    if IsAddOnLoaded("Totalrp3") then
        local UnitID = TRP3_API.utils.str.getUnitID(unit)
        if UnitID == nil then
            local name = UnitName(unit)
            return "|cff"..color..(name or "").."|r"
        else
            local name = TRP3_API.chat.getFullnameForUnitUsingChatMethod(UnitID)
            return "|cff"..color..(name or "").."|r"
        end
    end
end

However, it misbehaves.

If I target someone with a TRP profile, it works 100% as expected, however, whenever I target someone without a profile, or an NPC, it doesn't display a name.
The "if UnitID == nil" /should/ work around this issue, but for some reason it does now, and i'm confused as to why.