I looked early at this possibility but chose to implement it in C, for flexibility purposes as C is the raw pd programming api. But still, I took inspiration from the pdlua code where applicable and I actually wrote lots of the external in pure Lua, but instead of packaging it as external lua source files I embedded it within my C code as strings. I might be wrong, but I don't think pdtest will have problem with Lua paths... I compile and link Lua inside pdtest and then the Lua test suite files are loaded looking at pd's paths... so no need for Lua's paths at all.
L-P