cav, I daresay that's been done...
I'm really rolling with this! We'll see if it's wasted effort or not, but I've written about 8 pages so far of pivotal scenes (including some dialogue), all of the main plot, character's names, some secondary plot lines, some secondary characters, etc., etc. I plan to work on this a bit more until I have the foundation all laid out, then read a book in the line I'd be submitting to, tweak what I have, write the 2-page synopsis and 1-page query (have found some good query how-tos), and send it off to Harlequin! Will tell them I have 3 chapters ready, and will work on those 3 chapters while I wait for them to contact me and beg to see 'em.
(The standard seems to be that either you have the whole manuscript ready or 3 chapters.)