@kopfkopfkopfaffe: I am glad you ask. I just finished a write up of an experiment that exactly does that.
It is an improved version of sketch posted earlier in this thread.
It now handles autoreset and sets the baud rate of the uart.
It can be used to download sketches via the ide, but also to communicate with the sketch itself.
A minor hack in the core is needed though.
Here it is: