I managed to get the answer today,
- BAT ONLY in flight, it is NOT possible to Start the APU because the ECB-starter is INOP
- BAT ONLY on the ground it is POSSIBLE to start the APU (*if speed below 100kts)
The procedure Fcom Pro-Abn-24-Q elec Emer config takes into account the fact that the RAT is out, so the EMER GEN is working and the BAT ONLY procedure is non applicable because it is irrelevant. In that case starting the APU will not be an issue.