No idea if any or all of these parts are possible or not.
Browser plugin that pops up in the corner of your screen when it detects you have typed /meme in a text box
Captures focus and the rest of your text as you type
Has a button to "generate meme" or "cancel"
Possibly displays a typeahead suggestion like on Slacky's home page
If user presses generate meme (or enter) it will POST to Slacky and generate the meme
-- On response the text "/meme" in the text box will be replaced with the URL that Slacky returned
If user presses cancel (or esc) the popup disappears and focus is returned to the text box where the user started typing
Creates an implicit account on Slacky based on some unique signature provided by the browser to allow account-specific actions (custom meme templates, API usage tracking)
No idea if any or all of these parts are possible or not.