In PT10 with Production Toolkit, I use an Artist Series Control with soft keys for Preview, Punch Preview, and Write to Selected.
Before I got that, I used to use the Automation palette always in the same spot with a QuicKey macro (now I'd use a Keyboard Maestro macro) to push the palette's buttons from the keyboard. Clicking on the Automation palette buttons by hand isn't the worst thing in the world, but it does get tiresome after a while.
I can't remember if these buttons are even available in earlier, non DV or Production Toolkit versions of PT LE. But I can suggest something wacky as a workaround. You could use an AudioSuite version of your plug-in, it does the looping while you Preview and tweak away, then copy the settings of your plug-in and paste them onto the plug-in insert, or save the settings as a preset and recall them onto your insert. Then use write to all enabled, as has been mentioned. If you've already written pan or volume, etc, you can turn off those types of automation in the Automation palette so that you don't overwrite them when writing your plug-in automation. Good luck!