Oh yes, I've heard of this technique. It's a bit more complicated but once you get the swing of it, it's not too bad. It does requires being very familiar and very comfortable working with the Unsharp Mask tool in Photoshop but that's something everyone who uses PS should know anyway.
I do want to point out that bit depth (8-Bit vs 16-Bit for instance) is totally different and independent from Color Space (sRGB, aRGB, LAB, CMYK, etc.). If you're shooting and processing raw files and you're NOT doing so in 16-Bit Mode, you really need to be. You're not just shooting yourself in the foot processing raw files in 8-Bit Mode, you're blowing off your leg above the knee.
So, as I understand it, the process of Sharpening an image file while in a LAB color space would look something like this... Open the raw file in Photoshop and duplicate the Background layer. Then convert that layer to the LAB color space (Image -> Mode -> LAB Color). The logic behind this is that LAB separates the image data into three channels just like an RGB color space does, but... LAB breaks the image data down into the L channel, which controls Luminance, the A Channel which controls the Greens and Magentas and the B Channel which controls the Yellows and Blues. The important take-away here is that Luminance information is now completely separate from the Color information. The advantage is that you can sharpen the black and white pixels without any impact on the color pixels. There's a logic to that, too but it's early and I don't feel like diving into it unless someone's really interested.
Anyway... To do a sharpen while in a LAB color space you would go to the Channels tab and work on the L Channel ONLY. Sharpen using Unsharp Mask as you normally would. And no, you won't be able to use a High Pass Filter here.
I wouldnt mind having a look at that sounds interesting.
