Alter appearance of content types in teaser view through code?
The best example of this I can think of is the Read More module which adds a "read more" link to the end of the body teaser (in teaser mode obviously).

The idea is to use the node view hooks and teaser build mode to target what and when you want to edit.

For Drupal 7 you can look at the read_more module source for an example.
For Drupal 6 you can look at the ed_readmore module source for an example.