The way of hacking is similar to fmlsch. The differnce is the target to modify, which is FML::CGI:: class. FML::CGI::Calender is a sub-class of FML::Process::CGI.
If undefined method is called, fml loads it from FML::Process::CGI or FML::Process::Kernel. FML::Process::CGI uses CGI.pm to create a http header et.al. If FML::Process::CGI functions are not enough, FML::CGI class module should support the lack.
author's homepage is www.fml.org/home/fukachan/.
Also, visit nuinui's world :) at www.nuinui.net.
For questions about FML, e-mail <fml-bugs@fml.org>.