So here's the trick ...
You have have your existing relationship, leave it in place, but do not base any lookup fields on that relationships. It will be used for the "usual" drop down box / record picker data entry method.method.
Make a new data entry field called [Direct Entry]
Also make a new formula field called [Related Parent (final)].
The formula will be something like
IF([related parent regular method] > 0, [related parent regular method], [Direct Entry])
Now, make a new relationship to the parent based on [Related Parent (final)].
So then the use can use the drop down, or the direct entry and then the lookup will be off that formula result of those two fields. All your lookups need to come off the relationship using [Related Parent (final)].
