Streamlined and assosiations

I am trying to use Streamlined[1] in various projects and I am finding a
serious show-stopper: forms can't establish relationships. I mean, the form
at create-time (and edit-time as well) doesn't have the inputs to establish
the associations and the only way to do it is latter in the table.
This makes it impossible for me to use because my models validate that the
associations exists because they have to exist. I even have the relationship
at SQL level, so, with Streamlined, there's no way I can create new records.
Is there some way to solve this ? maybe by customizing the form (I am doing
that already with ajaxscaffolding, so, that's ok with me).
Any help is appreciated.
Thanks you.