Make computed field behave like a field type of my choice

Use case:
- I have a custom node type "Photo Series".
- Within this node type, I have a text field where users can define a subdirectory within sites/default/files.
- Someone is supposed to upload images into this directory via FTP.
- I want to use computed field to scan the files in this directory, and display them as a photo gallery.

Solution I:
The computed field spits out the html of the photo gallery. Most likely I will need additional computed fields for blocks with thumbnails etc.

Solution II (not possible, unfortunately):
The computed field behaves like a multiple imagefield, with values filled by php. Now I can use all the CCK formatters and modules available for imagefields (lightbox2, imagecache, etc).

-----

So, this is my feature request:
On the "Manage fields" page, I can add a new field with any existing field type I like (such as file, text, date, etc), and then specify "computed field" as the widget (instead of File Upload etc). Of course, as I write the PHP code, I need to take care to respect the format required by the field type.

---

See also
- CF should allow for other formatters (it's a D5 issue, so not a duplicate)
- insert image into imagecache with cck computed field