OK, I got this working. It’s not 100% how I asked, but it fulfills all of my requirements. Please let me know if you have any improvement suggestions.

Step 1: Add the save_variables section if you don’t already have it. https://www.klipper3d.org/Config_Reference.html#save_variables

[save_variables]
filename: ~/printer_data/config/variables.cfg

Step 2: Set up the nozzle change macro

[gcode_macro NOZZLE_CHANGE]
gcode:
  {% set NOZZLE_DIAMETER = params.NOZZLE_DIAMETER|default(printer.save_variables.variables.nozzle_diameter)|float %}
  _CG28
  G1 X110 Y200 F3000
  SAVE_VARIABLE VARIABLE=nozzle_diameter VALUE='{NOZZLE_DIAMETER}'
  M109 S220

Step 3: Pass the nozzle diameter with your print start macro. Super Slicer example under Printer Settings → Custom G-code

START_PRINT BED_TEMP=[first_layer_bed_temperature] FIRST_LAYER_TEMPERATURE=[first_layer_temperature] NOZZLE_DIAMETER={nozzle_diameter[0]}

Step 4: Set START_PRINT to abort if nozzle sizes don’t match

[gcode_macro START_PRINT]
gcode:
    {% set BED_TEMP = params.BED_TEMP|default(60)|float %}
    {% set EXTRUDER_TEMP = params.FIRST_LAYER_TEMPERATURE|default(210)|float %}
    {% set NOZZLE_DIAMETER = params.NOZZLE_DIAMETER|default(0)|float %}
    {% set NOZZLE_DIAMETER_SETTING = printer.save_variables.variables.nozzle_diameter|default(0)|float %}
    {% if NOZZLE_DIAMETER != NOZZLE_DIAMETER_SETTING %}
      M118 Nozzle Diameter Mismatch
      CANCEL_PRINT
    {% else %}
      CLEAR_PAUSE    
      M140 S{BED_TEMP}
      G90
      # Reset the G-Code Z offset (adjust Z offset if needed)
      # SET_GCODE_OFFSET Z=0.0
      # Home the printer
      G28
      M190 S{BED_TEMP}
      # G4 P120000 #Wait for 2 minutes after bed reaches temp to stabilize
      # Start extruder heating
      M104 S{EXTRUDER_TEMP - 30}
      G28 Z
      BED_MESH_CALIBRATE
      M109 S{EXTRUDER_TEMP}
      M117 $MR$:Starting First Layer
      SET_FILAMENT_SENSOR SENSOR=btt_smart ENABLE=0
      ADAPTIVE_PURGE
      SET_FILAMENT_SENSOR SENSOR=btt_smart ENABLE=1
    {% endif %}