Call c# script function from managed c# script in dll

In an effort to build a custom importer, I'm hitting a wall on a practical matter.

The basic idea would be to put all the basic import logic (several c# editorscripts) into a dll for practical purposes (1 file and user screw-up free), and have a user accessible c# script in unity that would be customizable by the user. More specifically, during import, I'd like to call a function found in that user customizable script to handle custom user defined properties found on objects. So we would have:

a DLL - Containing c# editor scripts handling the import of a file/object

a script - c# Editor script with a function we wish to call from the DLL. My customizable c# script looks like this:

 using UnityEngine;
 using UnityEditor;
 using System.Collections;
 
 public class UserPropHandler : Editor {
 
     public static void applyUserProperties (string pProp)
     {
         // write your user property handler code here
         Debug.Log("userData: "+pProp);
     }
 }

I have been trying to figure out how to call applyUserProperties from the DLL. I've tried reflection, which works when everything is in Unity as c# scripts, but once in a DLL doesn't seem to allow our function in the DLL to find the class from the script.

 System.Type upHandler = System.Type.GetType("UserPropHandler");
 System.Reflection.MethodInfo applyUserPropMethod = upHandler.GetMethod("applyUserProperties", BindingFlags.Static | BindingFlags.Public | BindingFlags.FlattenHierarchy);
 object value = applyUserPropMethod.Invoke(null,new object[]{nodeUserDataString});

I've also tried toying with the folders, but I'm guessing since the scripts are in a DLL, they are already compiled and thus where ever I put the c# script, it is compiled after.

Anyone ever tried this? Any suggestions?