From the ubl/mesh bl source
You should be able to use G29 S
* S Store Store the current Mesh in the Activated area of the EEPROM. It will also store the
* current state of the Unified Bed Leveling system in the EEPROM.
*
* S # Store Store the current Mesh at the specified location in EEPROM. Activate this location
* for subsequent Load and Store operations. Valid storage slot numbers begin at 0 and
* extend to a limit related to the available EEPROM storage.
*
* S -1 Store Store the current Mesh as a print out that is suitable to be feed back into the system
* at a later date. The GCode output can be saved and later replayed by the host software
* to reconstruct the current mesh on another machine.
*
* T Topology Display the Mesh Map Topology.
* 'T' can be used alone (e.g., G29 T) or in combination with most of the other commands.
You should be able to use G29 S
* S Store Store the current Mesh in the Activated area of the EEPROM. It will also store the
* current state of the Unified Bed Leveling system in the EEPROM.
*
* S # Store Store the current Mesh at the specified location in EEPROM. Activate this location
* for subsequent Load and Store operations. Valid storage slot numbers begin at 0 and
* extend to a limit related to the available EEPROM storage.
*
* S -1 Store Store the current Mesh as a print out that is suitable to be feed back into the system
* at a later date. The GCode output can be saved and later replayed by the host software
* to reconstruct the current mesh on another machine.
*
* T Topology Display the Mesh Map Topology.
* 'T' can be used alone (e.g., G29 T) or in combination with most of the other commands.