How to make the Open... dialog default to another folder?
The Open... Dialog defaults to the current folder for MOST applications. You chan change this by changing directory in Terminal (note that in my experience, some applications, especially Java ones do not follow this default behavior and in stead open the same directory every time, I assume this is hard-coded in the application). 

Now to the meat of your question: can we change this behavior on a per-application basis, using the tools OS X has pre-installed?  My guess is yes!, though I don't have time to do the research right now. What we'd be looking for is:

# wait for application to be switched 

# see if we have defined a default directory for that app

# if we have change, if not go to default 

# go back to top and wait to app switching


Others should definitely feel free to flesh out my skeleton code, please don't accept this answer until it is complete. I'll look again tonight and see if I can't finish that out. Potentially helpful link