As I understand it the problem is one of book-marking the question rather than having a convenient place to store the answer while it is being worked on.
A practical way of doing this is to post your first draft of an answer and delete it. Then all you need to do find the answer again is to search in the list of deleted answers in your profile. Hopefully this is much shorter than the list of non-deleted answers. When you are finished working on your answer you can undelete it.
This has the advantage that nobody comments on or (down-)votes your answer while it is still a "work in progress".
The disadvantage is that draft answers will be mixed up with those which you have abandoned altogether but cannot place in the trash can.
Alternatively you could mark the question as a favorite, if this is the only reason you use that feature. But this does not have the advantage of the delete feature.