Easy. You do it like this:

# Python
Application.PaintTool()

# JScript
PaintTool();

The real question is “how would you know about this undocumented command?”.

There’s several ways.

• Maybe you grepped through Application\DSScripts\*.vbs for “Paint” or “Tool” and found %XSI_HOME%\Application\DSScripts\tools.vbs
• Maybe you searched Application.Commands for the regular expression /Tool/ig.
• Maybe you searched the XSI mailing list (Paint Tool by script )