Solution
Bundle the Python script into an .exe file and call it using Start

Example

Create your Python script.

import sys
from openpyxl import Workbook

def write_data_to_excel(csv_data, output_path="output.xlsx"):
    values = csv_data.split(',')
    data = [
        {
            'region': values[i], 'sales': values[i+1]
            } for i in range(0, len(values), 2)
        ]

    wb = Workbook()
    ws = wb.active
    
    ws.column_dimensions['A'].width = 30
    ws.column_dimensions['B'].width = 20
    
    for k, row in enumerate(data, start=1):
        ws.cell(row=k, column=1, value=row['region'])
        ws.cell(row=k, column=2, value=row['sales'])
    wb.save(output_path)

def main():
    if len(sys.argv) < 2:
        print("No data to write!")
        sys.exit()
    if len(sys.argv) > 2:
        write_data_to_excel(sys.argv[1], sys.argv[2])
    else:
        write_data_to_excel(sys.argv[1])

if __name__=="__main__":
    main()

Use PyInstaller to bundle your Python script into an .exe file.
Copy the resulting folder to your GuiXT files folder.

pyinstaller write_excel.py

After installing, the Python script can be called from GuiXT.

GuiXT Script:

Set V[region.1] "Americas"
Set V[sales.1] "2584"

Set V[region.2] "Europe"
Set V[sales.2] "1228"

Set V[region.3] "Asia"
Set V[sales.3] "1628"

// The data needs to be put into a format the Python script can
// accept as a parameter
// This format ultimately depends on your Python script

Set V[rows] 3
Set V[row] 1
Set V[salesRegions] ""
label loop
if not V[row>&V[rows]]
  if not V[row=1]
    Set V[salesRegions] "&V[salesRegions],"
  endif
  Set V[salesRegions] "&V[salesRegions]&V[region.&V[row]],&V[sales.&V[row]]"
  Set V[row] &V[row] + 1
  goto loop
endif

// Ensure that the path points to the location of
// the executable you created with PyInstaller
Start "path/to/write_excel.exe" parameters="&V[salesRegions]"

Components InputAssistant + Controls