Engines, Rails App architecture in OpenSource context


I have posted the following message on Rails Engines forum but as there
is no activity their (strange isnt it?) I re-post it and extend it here.

Here we go:

[Engines, Rails App architecture in OpenSource context] EXTENDS
[Modularity, engines and loading:

I am working on blankapplication.org (an OpenSource CMS 2.0 for the
moment but a service mashup plateform slowly slowly). We would
like to add an extension behaviour to it as it is done on Spree. I
thought initially to plug our new addons using engines but I came to a
loading problem: how can I override controllers/models/views from the
main app/ by those present in the engines. The native behaviour is just
the opposite: you override controllers/models/views from the engine in
the main app/.

As far as I have understood on Spree, the kernel is also embeded in a
Spree Extension (kind of engines in fact). Consequently, when you tune
your Spree you can make it directly from the main app/ folder and
....override engines parts ... or simply add you own extension.


Does anybody have an idea about that. Some french ppl told me
(http://www.ruby-forum.com/topic/201695#878314) to follow that way,
putting BlankApplication in a home-made engine stuff. Radiant and typo
also seems to go that way.

If anybody ever used/install/try them or as been through this "Extension
topic", could he give me some keys (links, book, use case, plugins or
just "Hi man, you are on tricky subject. All the best!!!")