There was a 6-shot .40S&W L-frame - the Model 646. No reason the slightly longer 10mm cartridge couldn't also fit in an L-frame cylinder, but it IS a higher-pressure cartridge -- 37,500 psi for 10mm, vs. 35,000 psi for .40S&W, according to Wikipedia (interestingly, .357 Magnum is also spec'ed at 35,000 psi according to Wikipedia, and of course there are L-frames in .357 Magnum -- the 386, 581, 586, 619, 620, 681, and 686).
While I do have a Model 610(-3), I'd also love to see the 610 reissued (again).