I'm a sucker for punishment.
Or, I noticed that FreeBSD's pkgng binary package repository ships with a port of the Arduino development environment. It's this java thing that wraps around avr-gcc and avrdude. It's very popular, it's open source, and I figured what the hell.
I plugged in my Arduino Leonardo and .. it was detected as a umodem device. Excellent!
.. and then it wasn't. It went away very quickly and came back as a single interface (OK) with three child interfaces (Hm, okay), but only one uhid (human interface) interface active (Not Ok.) The modem port used to program and talk to the thing wasn't there.
I then went on a bit of a journey. I found that quite some work had already been done to correct issues in the FreeBSD USB stack - however, it still wasn't working. It showed up fine - it identified itself as a generic USB serial port device, and yet umodem didn't bind to it.
Next - the umodem source code. It yes, claimed anything identifying as a USB serial class device - but it only claimed devices that ALSO identified as an AT-class modem. Yes, a serial modem that you speak AT commands to. The Leonardo identifies itself as a USB serial class device but with NO command encoding. umodem didn't like that.
So, to the USB 1.1 standards documention! After reading the relevant bits, I discovered that the rest of the device handling is the same! Ie, it doesn't matter whether the device says "I speak AT commands" or "I speak no commands", it's still serial. This identifier is just for the upper layer application to decide whether to send AT commands or not.
Thus the fix was simple - also claim devices that say "no commands" as well as "AT commands." That fix is in -HEAD and I hope to try and sneak it into 10.0.
And with that - FreeBSD-HEAD is now a viable development environment for the Arduino Leonardo.