Okay, here is the little bookmarklet -- it's pretty limited, but it serves my own laziness, so I share it FWIW and if anyone has the time and wants to upgrade it, go for it and drop me a comment and I'll (ha ha) signal boost any new versions!
And the first comment on it was somebody hoping people would check for access lock when using it, and, well, my coding fingers were itchy (they get itchy about every six months, which is not actually often enough to build skill, sadly), and astolat gave permission to modify, so here's a modified version that automatically warns you if you're about to link to a locked DW post. (I think it would probably also work on locked LJ posts but I haven't accepted LJ TOS, so I can't test that.)
All you do is log in to DW, go to the page you want to signal boost, highlight any text you want to quote, and then click on the bookmarklet. If it's access-locked, you'll get a pop-up warning you and asking you if you want to continue. On any other webpage, it will automatically redirect to the DW posting window.
To set it up, make a new bookmark, and copy-paste the following as the URL:
(If I get around to it, I may figure out how to make it open in a new tab, too. If so I'll update this post.)