I downloaded sudser (without installing it) and had a look at the sudser-worker script, which tries to add a line like:
where $shells is a comma-separated list of the commands (shells) found in /etc/shells.
Problem is that (1) doesn't quite make sense, and (2) you don't have that file.

I happen to have that file because screen and tmux put themselves there. Normally /bin/sh should be there (you can add it there).

In any case this tells us that:
(1) sudser is buggy and unsupported
(2) you have to fix this yourself
(3) the sudo-situation in Maemo is horrible, only on-par with the dependency hell we have.

If Maemo didn't brick whenever you *think* about modifying a critical file I would propose replacing "update-sudoers" by a dummy and keeping a user-controlled list that nobody can update.

As we have it now (answering your security concern) every "app" can (1) do rm -rf / on postinst before you can say "huh?" and (2) add a file to /etc/sudoers.d to give itself root rights (many apps do this, and it doesn't seem to be considered unpolite by the people here

I think there's been already at least one thread or sub-thread dealing (= talking about, not solving) with the sudo issue. This is our own version of the "elephant in the room", but we just love our elephant the way it is.

So not only do you have to keep the N900 close to your heart, but you also have to be (100-N)% sure and N% naively-trusting about what you install.