I was also thinking about something similar. I'd like to print a part that would have 2-3 structural walls, filled with loose gyroid infill (10% or less). Then I would like to fill the interior with polyurethane or epoxy resin to create a PLA-reinforced resin matrix. It could save dozens of hours per part over printing with 100% infill, and would likely increase strength in the z-direction since the internal structure would end up as one homogeneous chunk of plastic instead of fused layers.
My current solution is also to use a drill, but that is error-prone, inconsistent and requir