Yes, if you have installed Teensyduino, then your Arduino IDE has been patched to recognize those directives in your boards.txt file. They are not limited to Teensy in any way. You are welcome to use them.
The modified java source code is copied to a "src" folder within your Arduino folder. On a mac, control-click on Arduino and select "show package contents". On Windows and Linux, you can see inside the Arduino folder as normal files.
I submitted this as a patch the developer mail list on December 23, 2008 and again January 1, 2009. It was rejected on January 3, 2009, with the following explanation:
This is a small patch but a big change in philosophy. The Arduino IDE isn't a general purpose AVR development environment. It's a tool to support the programming of Arduino (and Arduino-compatible) boards. Adding the ability to launch arbitrary external tools would change expectations for the software, suggesting further complex additions that would add little value for most Arduino users.
If there's interest in a general purpose AVR IDE, might it make more sense for that to be a separate application? It could, of course, build on the source code for the Arduino environment. Additionally, the Arduino core libraries are standard AVR C and C++ and usable independent of the IDE (e.g. with the included Makefile).
If this sentiment has changed and this patch would now be welcome in Arduino, I would be happy to prepare another patch against the latest in github. But unless David speaks and specifically says this patch is welcome, there isn't much point going to that trouble.
If anyone wants to use these directives, separately from Teensy and my Teensyduino installer, I'll be happy to help (eg, separate those bits of source code from the other Teensy-specific patches). Even though the GPL only requires providing the source, which the installer does, if you need a bit of help, all you need to do is ask. Of course, be prepared to compile your own customized Arduino IDE (eg, JDK, ant, misc dev tools, etc). This stuff isn't standard in the Arduino IDE, and unless opinions have changed since January 2009, it probably never will be.