Users have reported that the shortcut prefix keys, CTRL+ALT+V (on Mac OS X, Command+Option+V) does not work in Gmail's message editor. Pressing those keys have no effect, or would cause a beeping sound.
As explained in a post to the AE Creations Help and Support forum, the issue lies with Gmail filtering out these keys when the cursor is inside the message editor; it doesn't happen when the cursor is in the To or Subject fields. I experimented with changing the Clippings keys using the KeyConfig extension (e.g., assign CTRL+F4 to the "Insert Clipping" action), but that did not work.
Interestingly enough, other Google products that I've tried are not impacted by this issue -- specifically, Google Groups (when writing a forum post), Google Chat (when typing a chat message) and Blogger (when writing a blog post such as this one).
My only suggestion would be to submit your comments to Gmail support. Sorry.