How can I make individual nodes fieldable? One of the great things about the Entity and Field APIs is the ability to add fields to any entity (nodes, users, etc.) and subsequently any bundle (e.g. node content types).

But is there a way, using the Field API or the Entity API, to make fieldable nodes? That is, have field instances attach to individual nodes instead of content types/bundles. So node 1 might have a file field and a text field, node 2 might have a number field but no text field, etc.

If there isn't, is there any work being done using other concepts that would achieve the same result? The closest I can think of is Webform, but I'm sure that it's not intended to be a way to create content, and it doesn't take advantage of the Field API for its fields.
You could use something like http://drupal.org/project/conditional_fields but that's all done from a front-end point of view, which means Drupal doesn't really know about your structure.

http://drupal.org/project/properties looks interesting and may suit depending on your needs.

I had a similar issue recently and decided to split up into three separate content types instead of just one.