TM_SELECTED_FILES appears to be pretty close, but it only works on "selected" files within the project. Is the list of shell variables in the manual a comprehensive list? I'm hoping there's a secret something that I've missed :)
Well there is TM_PROJECT_DIRECTORY or TM_DIRECTORY. Neither is quite perfect, but might work in some cases.
http://www.macromates.com/textmate/manual/ environment_variables#dynamic_variables
Perhaps we'll get a more relevant variable as the project window is redone in the coming months.