Using xampp to modify the webgui will not work for the following reasons:
1) M0n0wall uses a pretty old version of PHP
2) M0n0wall's core is deeply integrated into many PHP files that are also required for the webgui, you'd have to simulate their behaviour
However, there is a solution for this. Just upload the changed PHP files onto m0n0wall and execute the following script:
chmod a+rx /tmp/*; mv /tmp/*.php .