I'm actually talking about sort of the reverse. Secure Input is a system-level thing; it disables input capture via the accessibility API (which things like TextExpander use), which effects everything on the machine. That is what you want; when typing a password, there is no good reason for other code to be reading what you (or LastPass) is typing. You just also want it to be turned back off when you're done.

But after the first use of the LastPass Safari extension, TextExpander (or KeyboardMaestro, or a bunch of other tools) won't work anywhere until a after reboot or some other method of disabling SI.