A good strategy for Jade template inheritance with external modules, on Express

I'm working on a Framework where developers can implement a child template in an external module. I would like to include later that template in a parent template.

Here is a basic template schema :

─ mission (parent template)
├─ succeed mission (child template)
├─ failed mission (child template)
└─ play mission (child template)

The parent template is the same for everyone. The children templates are implemented by a contributor and always displayed in a different moment (mission state).

I try two approaches in a single template file.

Implement a "block" pattern :

block mission_play
  h2 Hello world!
  p Lorem ipsum dolor sit amet, consectetuer adipiscing elit.
block mission_succeed
  h2 Congratulations!
  p You won buddy!
block mission_failed
  h2 Oh my gosh, dude, you did something realy bad
  p You should try again

That one won't work because I haven't got any extend layout directive (that I can't use because every child template is an external module). Is there any way to indicate manually to jade which template extend ?

Implement every child template in a mixin:

mixin mission_introduction(mission)
  h2 Hello world!
  p Lorem ipsum dolor sit amet, consectetuer adipiscing elit.

mixin mission_succeed(mission)
  h2 Congratulations!
  p You win buddy!

mixin mission_failed(mission)
  h2 Oh my gosh dude, you did something realy bad
  p You should try again

I was going to include that template in its parent but Jade disallows usage of expressions in the include's path. That why I try to interpret the jade code directly in the parent jade template but I didn't find how. I even tried to include it with the partial function but I can use that module without disable the default views inheritance strategy in Express.

So where I'm wrong ? What is the best strategy for that king of architecture ?

Cheers !