To create full list of script commands, you need to switch "verboseLogging" in options.cfg to "true", then launch oxce, then quit and open file "openxcom.log". It will contain not only log, but script help. Save it somewhere, and remember to update when upgrade to newer OXCE version.
edit: what is the correct synthax to get/set transform_chance?
Just as any variable,
set temp transform_chance;
...magic...
set transform_chance temp;
Like 0 is no transform at all, 100 - transform by any circumstances.