There is no v5 utility for ML.. that includes the archive but it can be slipped into the OS with a little magic. It is compatible but the installer isn't. See below.
If you want to copy the files you are better using a backup utility like CCC or superduper that should be able to copy the TC drive to a local external disk.. that would be much faster than USB drive plugged into the TC, albeit not so convenient.
How to load 5.6 into ML.
1. Download 5.6 for Lion.
Click to open the dmg but do not attempt to install the pkg.. it won't work anyway.
2. Download and install unpkg.
Run unpkg on the desktop.. it is very simple.. drag the AirPortUtility56.pkg file over to unpkg.. and it will create a new directory of the same name on the desktop.. drill down.. applications utilities .. there lo and behold is Airport utility 5.6 .. drag it to your main utilities directory or just run it from current location.
You cannot uninstall 6.1 so don't try.. and you cannot or should not run them both at the same time.. so just ignore the toyland version.. the plastic hammer.. and start using 5.6.. a real tool.