I've done a bit more digging, and the error is caused by there not being a case for 10.7 in the makeuser file. I have reported it to Apple on the feedback page, but in the meantime I managed to fix it for myself.
It's a little tricky to correct this because the package is a flat package so you can't right-click and choose show package contents to edit it, you have to use pkgutil to expand the package, edit the makeuser file and then flatten it again.
Copy the Custom Remote Desktop Installer.pkg to your Desktop.
Use pkgutil to expand the package, open Terminal and enter:
pkgutil --expand ~/Desktop/Custom\ Remote\ Desktop\ Installer.pkg ~/Desktop/Custom\ Remote\ Desktop\ Installer
Open the Custom Remote Desktop Installer folder that has been created on the Desktop, and right-click on RemoteDesktopClient.pkg and choose Show Package Contents. Then open Scripts and then Tools. Then open makeuser with a text editor.
Find the section in makeuser where the cases for the different OS versions are set. Add a condition for 10.7, the section in question should look something like this after you have changed it:
10.6*)
os_version="6"
ucmd="dscl"
ucmd_users="Users"
ucmd_groups="Groups"
;;
10.7*)
os_version="7"
ucmd="dscl"
ucmd_users="Users"
ucmd_groups="Groups"
;;
Save makeuser (making sure that the name and permissions are preserved).
Now flatten the package:
pkgutil --flatten ~/Desktop/Custom\ Remote\ Desktop\ Installer ~/Desktop/Custom\ Remote\ Desktop\ Installer\ 2.pkg
You will have a new package on your Desktop, Custom Remote Desktop Installer 2.pkg which you can test on your client Macs.
This worked for me, as always, YMMV. Good luck.