This is not easy unless rim decides to distribute the SW in a different package.
Currently, this is a setup-launcher from Macrovision.
To extract, you have to use a Windows OS (maybe, wine is ok)
After installation, you find the firmware files in:
\Programs\Shared Files\Research in Motion\Shared\loader files
If you copy that into the (currently unknown folder) where the online update which bbdm puts the files into, you should be able to perform an update.
MY guess is, this folder might be in ~/Library/Application Support/BlackberryDesktop/????
To find that out, you could use an old, spare blackberry which you know an official updat exists, then run the online update and search, where the loader files have been downloaded to on your disk.