And how about:

&dtml.missing-REQUEST.form['foo'];

That is, allow expressions to be used. I guess this makes the entity syntax act more like <dtml-var>.

This would allow a really clear syntax for refilling <input type=text> tags with previously entered values in a form.

You can explicitly select a namespace (and exclude others) using <dtml-with ... only>, for example: <dtml-with expr="REQUEST.form" only> <form> <input .... value="&dtml.missing-foo;"> </form> </dtml-with> If you don't use the "only", this still enforces that the REQUEST.form namespace is the first searched.