The idea is similar to Spaced repetition for reading inbox, but applied to § Zettelkasten-writing inbox.
If you want to write about something, add a note to the writing inbox
Use spaced repetition to schedule writing, the same way as it is done for reading
If you feel like you start forgetting the topic, write it down and make flashcards or delete it completely
Review the writing inbox when you feel like writing
This writing inbox can be also used to review some fruitful notes to see if you have something to add.