I agree that this would be nice for Snippets defined via the “Inline (Snippet)” resource type within the page — it would be very difficult to pick up on the names of Snippets defined in Static Resources referenced by the page, but picking up on Inline (Snippet) resource names would be a good start.

Either a separate parameter to determine if the snippet is in-page (which would render the picklist), or the picklist has an “other” option which would render the text box.