However, I just realized that the extension just opens a URL and pastes text into it; one can easily open that page URL in any browser, and paste any text you wish into it:
http://toolbar.google.com/send/sms/index.php
Enjoy!
[robg adds: Google seems to be having a little bit of server trouble this morning; I tried both Safari and Firefox (via the plug-in) and only saw "Server error - try again in 30 seconds" each time I tried to send a message. Still, it seems it should work just fine from Safari once the server starts behaving again...]

