There is no way to do this, "launchd" is the master process, the equivalent of "init" in other unix systems, the only way in any other daemon would be to perform some sort of kill -HUP to reload config files etc, but this operation is not permitted on launchd. If you look it up in the process table it is the number 1 process, every other process that is currently running is in some way a child of it, so a reboot is the only way to achieve this. You may have further luck in setting those environment variables in another area that is more inclined to allow restarts.
Dans-iMac:etc stuffe$ ps -ef | grep launchd | grep -v grep
0 1 0 0 9:40am ?? 0:09.47 /sbin/launchd
65 18 1 0 9:40am ?? 1:53.64 /usr/sbin/mDNSResponder -launchd
213 73 1 0 9:41am ?? 0:02.55 /System/Library/PrivateFrameworks/MobileDevice.framework/Versions/A/Resources/usbmuxd -launchd
0 85 1 0 9:41am ?? 0:00.08 /System/Library/CoreServices/ODSAgent.app/Contents/MacOS/ODSAgent -launchd
89 230 1 0 9:41am ?? 0:00.01 /sbin/launchd
501 238 1 0 9:41am ?? 0:00.23 /sbin/launchd
501 933 238 0 10:53am ?? 0:00.01 /System/Library/CoreServices/AirPort Base Station Agent.app/Contents/MacOS/AirPort Base Station Agent -launchd -allowquit
Of course, missing the obvious solution that you can add the relevant variables into the config file, and then just run the commands manually in order to escape the need for a reboot, nothing that only newly start programs will take account of any changes. I found this page which explains it in more detail:
Environment variables set in launchd will be available to all programs on the system. However since $HOME/.launchd.conf is not
supported, this approach is used for global environment variables
The /etc/launchd.conf file contains a list of commands to be run by
launchctl during startup. However environment variables set in
/etc/launchd.conf cannot have spaces and cannot reference other
environment variables; launchctl when run in Terminal does not suffer
these limitations. Do note that /etc/launchd.conf does not exist by
default, so we will need to create it.
A reboot is necessary to load the contents of /etc/launchd.conf.
However if you were to run the corresponding launchctlcommands in
Terminal, you can alleviate the need for the reboot. Just remember
that if you only do the launchctl commands, and do not put the entries
in /etc/launchd.conf, it will not persist after reboot.
If you want to have a system wide environment variable that uses
spaces or be dependent upon another environment variable, you can put
the launchctl commands into one of the files Terminal reads. You will
have to quit and relaunch Terminal to access the newly set variables.