pretty crude XML here that I just made while waiting for my pizza to bake, pretty sure you insert this into a random XML and it will work, or at least give you an idea.
Check Windower as well, there has to be one uploaded there.
Code
<if Spell="Wildfire">
<if advanced='("%WeatherElement" = "Fire" OR "%DayElement" = "Fire")'>
<action type="equip" when="precast" set="fire-obi-set-here"/>
</if>
<else>
<action type="equip" when="precast" set="normal-belt-set-here" />
</else>
</if>
PS - I'm a really old school spellcast user who hasn't had a real need to upgrade to newer stuff. I'm sure there is an easier way to do this by now so maybe someone else will chime in here for you.