You can use FTL with probably just about any boxing software.
As long as it supports the equivalent of hotstrings/keymaps.

If you can push 1 on a window, and have it send 1 + a mod key to the other windows/sessions.
Then you can run FTL with it.

The first FTL set ups were with Keyclone.
I know you can do it with HKN too.
I couldn't get it working with KC and have never used HKN.

With IS Boxer, it uses the same method as KC, except its behind the scenes.
That was a large selling point for me.
Since FTL is really nice, but not that easy to set up.