Looks like you have this solved, but if you wanna throw one together yourself, I just did this this last night. It rests in the entityclasses.xml
Code:
<config>
<!-- Changes decay time before zombie loot containers disappear, in seconds, default 5 min (300 sec)-->
<set xpath="/entity_classes/entity_class[@name='EntityLootContainerRegular']/property[@name='TimeStayAfterDeath']/@value">600</set>
</config>
I haven't actually timed a loot container yet, but it doesn't throw any errors or warnings at me in the console so I'm assuming the code is correct and doing its job.