Answering myself... the following command opens the WLAN wizard (at least on the N8), so the user can manually stop the current access point:
After that I can use the btsocket module to start the adhoc connection, BUT: I must use the socket module instead, since I need the settimeout() function (not available in the btsocket module). If I try to use both modules at the same time, I get a 'Unknown error: 142' when I bind the socket. To work around this, I wrote one app to just start the adhoc AP (using the btsocket module) then launch another app with the remaining of the app (using the socket module)... There was no error during the socket bind this time, but no data seemed to go through the socket.
e32.start_exe('wlansniffer.exe', u'', 1)