signup for non users ?

I'm using the signup module to allow people to subscribe to trips. One requirement is that
the trip leader can edit/add/remove signups for their own content. The feature that I'm after
is an edit link associated to each user that signed up for the trip (to edit the signup info)
and an "add signup" link to allow the trip leader to add somebody to the list even if it is not a user.

The edit link is a new feature (I think).

I'm not sure if I can implement the "add signup" link easily. If I'm not wrong, there is a
restriction of one signup per user. Therefore the trip leader won't be able to add only one
signup (for himself). Would it be possible to lift this restriction (or make it configurable) ?
The default behavior can then be restored by checking if the user has already signed up
in the submission form...

Does this make sense ?

A related question: Is there any particular reason why signup details are not nodes ?
Having the sugnup details as a node (maybe using the subform module) will open up all sort of possibilities...
For example the list of signups for a content will be easily done with a view, the signup form will be
a cck type. The edit/remove feature will be there for free...

p