I was able to load the object from the library, calculate dynamics and render it and save it back to the library without losing the external mesh<<
Great! One step closer. I must have typed something wrong in my test.

the moment you perform any manipulations in the hair room, the hair becomes embedded again.<<
I think one can live with that. This is more a distributing issue. Once it is on the computer all is ok, I think.

The goal is to be able to have the original MilCat obj path for the cat. And the path for the hair pointing to the hair obj file. Like this:
{
version
{
number 5
}
figureResFile :Runtime:Geometries:DAZAnimals:blMilCat.obj
actor BODY:1
{
storageOffset 0 0 0

--- snip ---

hairProp Hair_1:1
{
storageOffset 0 0 0
objFileGeom 0 0 :Runtime:Geometries:Tiny:hair.obj
}
{
geomCustom
{
numbVerts 6160

---

That is what I tried for the frog. Will test more later.