Alas,
"we are sorry, but at this time you may not ...."
How many times have I seen that message in the last 14 years. Too many.
So the only solution is to make the reference field in the relationship not be dependent on a lookup field. The way to do that is to create a new field called say [Related Parent populated by Action (or Automation)]
ie, you need to mirror that ineligible fields with a pure data entry type field maintain the field to be equal to the original field with an Automation or an Action..
So for example, you can make saved table to table import with a filer on where those two fields do not match. Then create a saved table to table import, Merging on [Record ID#] to map the one field into the other, subject to the filter that the two don't match.
Then set up an Automation or you may need more than 1 to trigger on whatever edits or creates which might cause those to not match.
