Maybe I can provide answers to that question... at fr.ws we now have
lots of contributions to the "Page:" namespace, and I think that we have
a good experience on how to teach beginners on that.

I guess the hardest concept to explain is transclusion ; this should not
come as a surprise to you, since you pointed out the existence of a
"Page:" namespace as an obstacle.

The solution is simple : just forget about it, do not explain it to
beginners :-) . It is something they do not need to understand right
from the beginning ; they will discover it later.

Instead of trying to explain transclusion to beginners, we redirect them
to the "Page" and "Index" namespaces, right from the beginning. And it
works : many new contributors make their first contributions in the
"Page" namespace, without knowing anything about transclusion.

How do we do this ? Of course, we use the "help" pages, but also :
*we use the "welcome" message
*we have a "contribute" toolbox, visible from everywhere, with a "random
book" link that leads to a random index page.
*we have categories of Index pages sorted by progress, and these
categories can be reached from the RC page.

Of course, this comes at a cost : New users who started to contribute in
this way do not always handle hyphenated words correctly, and they
sometimes write the page header in the main box. But this is something
they learn quickly, and it is a minor problem compared to the benefits.

Another difficult thing is the upload of djvu files and creation of
index pages ; this is a bit difficult for beginners. However, if index
pages are already created by more experienced users, then beginners can
start proofreading immediately, and this allows them to get a taste of
Wikisource. So, create as many index pages as you can. One of our
contributors spends most of his time creating new index pages, and I
believe that we owe him a lot of contributors. I also create an index
page everytime I find a text that does not have scans. In addition, we
are planning to create 1416 new index pages very soon with a robot,
following an agreement with BnF/Gallica.

So the conclusion is :  create lots of index pages, and expose them to
as many beginners as possible.

Thomas