I like your proposal, and I think that at a minimum, they could change the shield icon in the context menu to match the browser extension icon (with the padlock overlay when the extension is locked).
Presumably, most users who routinely autofill using the right-click context menu have disabled the inline autofill menus. Thus, in the context of this feature request, the display of the lock status in the inline overlay icon does not help.
Although I do not use inline-autofill, I do have it display the shield as a reminder that my preferred autofill (control-shift-L) is a viable option at this point.