I know this will not really answer the question, but actually it does not look possible to do this.
The best way I found to achieve this goal (1 code -> destop + mobile) was to make 2 projects 1 for mobile, 1 for desktop, sharing the same single library full of my app classes + sharing the assets, etc... The only thing that cannot be shared is the descriptor xml file + base class file (though setting up those files is a one-time duty).
A good illustration of how things work can be found on Chris Campbell reversi project page : checkout the last screenshot in his post right here
I hope this helps...