You're going to have to set it on a shell-by-shell basis; Bash and csh-like shells do not share the same configuration files and syntax for adjusting the PATH.
Trying to do this in launchctl will not work, because environment variables are set on login; they do not exist system wide in Unix outside of a shell session.
So you're going to want to add
setenv PATH "$PATH:/add/my/extra/path"
to /etc/csh.cshrc and
export PATH="$PATH:/add/my/extra/path"
to /etc/bashrc.
If you want environment variables in GUI applications, that's more complicated. You have to create a .MacOSX/environment.plist file in each user's home directory. The .MacOSX directory will likely not exist by default, so you'll have to create it.
The format of the file is like so:
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple Computer//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
<key>PRINTER</key>
<string>myprinter</string>
<key>PATH</key>
<string>/path/to/thing/I/need</string>
<key>DISPLAY</key>
<string>0:1</string>
</dict>
</plist>
More on the environment.plist is on Apple's site.