RE: DX7 TX Charge/Discharge
You should have no problem Discharging or Cycling your batteries through the Tx Charging Jack.
Older generation Tx's had Diodes installed to prevent Polarity problems. This would also prevent you from Discharging, Cycling, and using an ESV, through the Tx Charging Jack. This is not a problem anymore. Tx Mfg's use a different method to protect Tx's from charging with the wrong polarity.
In the past, we used to solder a Jumper wire across the Diode to allow us to Discharge and Cycle Tx batteries in the Tx. Charger Mfg's used to publish Instructions and Diagrams to show how to do it.