Stupid error (as always). Wrong file path provided
My thread was blocked as in juce_ConnectedChildProcess (line: 154) childProcess's start returns true whereas the slave executable is not properly launched...
if (childProcess.start (args))
connection = new Connection (*this, pipeName);
sendMessageToSlave (MemoryBlock (startMessage, specialMessageSize));
connection = nullptr;
...and so the write in the pipe waits for the reader for ever.
That's pretty naughty that childProcess doesn't return false in that case (and TBH i didn't investigate further to know why).