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NOTE: This is really just an answer to how I implemented Tom's answer, but I needed to post an image so couldn't just comment.

Ultimately I tried options #1 and #2, and ended up using #1. Attached is the final transform tree graph that resulted in usable transforms. At startup /rex_ground_calib will calculate an initial offset that will make the ground flat (using ground plane detection), and then dynamically broadcast a transform that maintains this as the zed_tracked_frame moves relative to this by dynamically linking zed_tracked_frame and base_link so that everything offsets correctly.

Zombi answer inspired by Tom's comment here: Github Page