Defender: Outside of the wrist on the hammer arm, and under the boiler (as you mentioned). Possibly something in a brace on the shoulder (extra gun ammo for long campaigns maybe?) but that wouldn't look convenient for trenchers to access. Other than that... not really anywhere, as it would look over loaded for "battle" I think.
Hunter would be difficult, given that he isn't that big anyway, without looking a bit silly. A couple of shovels or a pack down the side of the boiler at the back, but even on the arm, strapping stuff to it would seem odd, I think.
Jacks, just generally don't look like they're made for hauling baggage AND fighting at the same time, for the most part. I figure if they were being used for haulage (which would be rare, but foreseeable), they'd have a cart or sling attached to their two shoulders (see the rings on the defender), and haul, then remove the baggage when moving to battle, so as not to be overloaded. Even more than a few strapped on extras would make them look off balance, is what I'm saying, I'm not thinking they'll be carrying the entire platoons packs

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Oh, consider a large "shovel head" on the arm of the Defender, and say it's interchangable with the haft of the shock hammer, to be used as a trenching tool maybe?