I think you're (implicitly) asking three questions at once, so I'll address them individually.
***
### The question in your title

Q: Is it possible to search sister SE sites before posting a question?  
A: [Yes](http://www.google.com)

Honestly even if you were on the Webmasters site your question may not have pulled up the one we linked you to. The Stack Exchange site-search is pretty awful - you should *always* ask Google first (considering it is basically the sum of all human knowledge these days).

That said, even Google fails sometimes - there's nothing wrong with googling, not finding what you need, and asking anyway. The worst thing that can happen is your question gets closed as a duplicate (or "off-topic, go look here instead").

It would indeed be nice of asking a question on one SE site searched all related SE sites, but that idea has two major problems:

1. What is a "related" site? (nobody is going to want to curate that manually), and
2. ***Search is HARD***

(If you want to raise that suggestion on [mSO](http://meta.stackoverflow.com) you can certainly do so, but it's probably already been suggested and shot down.  mSO is also not the most hospitable part of the Stack Exchange network...)

***

### The question of topic fragmentation being harmful

This has been brought up and debated to death many times. A lot of us feel topic fragmentation is a Bad Thing, myself included (if you think Webmasters has a lot of overlap with Server Fault consider that 100% of [unix.se](http://unix.stackexchange.com) is covered by Server Fault and Super User, which both existed well prior to unix.SE).
  
Unfortunately the net consensus is that specialization trumps fragmentation.  
Maybe one day we'll reverse that.

***

### The feature request to allow `the community, as well as the moderators were able to exert some say on whether questions are right for a particular forum`

Is [tag:status-completed], [tag:status-bydesign] (through the [Reputation and Privileges](http://serverfault.com/privileges/close-questions) system).