re: Grisaille and any theme with an active excerpt box you can use this workaround
You compose your whole post first and before you publish it you switch to the HTML (Text) editor, copy the code, paste it into the Excerpt box, and publish the post. http://en.support.wordpress.com/splitting-content/excerpts/#creating-excerpts
If you do not see an excerpt box then go to screen options on the top right hand corner of your Admin page and include:
Post – Likes and Shares, Format*, Categories, Tags, Excerpt, Send Trackbacks, Discussion, Comments**, Slug, Author, Revisions, Writing Helper
P.S. On themes that do have an active excerpt function, the workaround works very well but you have to do it on every post you create.