I'm pretty sure I managed to do that like a year ago or so.
When disabling vanilla health and using the overlay to display each body part separately it only appears when taking damage/regenerating. Is there a way to always display the overlay? Because I took this screenshot and I don't know which version it is https://prntscr.com/wj013l I'm currently running 1.6.14 (RLCraft).

Try to make the X-axis offset value smaller, for example 10

yes, there should be something like visibleDurationTicks in the overlay config. Set that to -1