Beh! Looks like it is currently not possible to use debsign from etch to sign packages built in a sid cowbuilder environment.
Update: Adam just pointed out that this was already announced on debian-devel-announce. He proposed to get an updated version of devscripts from etch-backports.
I wonder if we shouldn't provide a fixed version of devscripts in etch officially, just in case someone who maintains packages actually *erm* uses it.