I will try the mixing of channel 2 and 5 (which is aux 1). The battery goes into channel 6.
If you're Tx needs channel 6 for left aileron (flaperons) and you only have a 6 channel receiver, just plug the battery into any Rx slot that is available. If channel 5 is not being used, plug the battery into that slot and let your Tx function like it was meant to for flaperons, no custom mixing necessary.