This is possible since Roundup 1.4.17:

changeset:   4481:9bbf3758c16a
user:        Stefan Seefeld <stefan@seefeld.name>
date:        Thu Apr 14 12:47:48 2011 +0000
files:       roundup/cgi/templating.py roundup/hyperdb.py
description:
Allow default value declaration in DB schema.


The value is declared with like this in schema.py:
  value=String(default_value="something")
  status=Link("wpstatus", default_value=1))