Think of a workaround for him:
if (Regex.IsMatch(srcDir, @"\\[^\\].*"))
srcDir = "\\" + srcDir;
srcDir = "\\" + srcDir;
Had not noticed that a guy has blogged that: http://blogs.msdn.com/helloworld/archive/2008/04/08/customactiondata-and-user-defined-path.aspx
But the installer class and CA is really helpful to those DTG guys, I may later blog further about it when at free.