QTR-Quadtone RIP

Without a decent densitometer, it's pretty hard to build an ICC
profile that will give you good WYSIWYG between the screen view and
the actual print.  I have found, however, that I can do a pretty good
job of this by creating a custom dot-gain curve for gray-scale images.
 While this method doesn't do anything to show the relative warmth or
coolness of the end product, at least it will give me a reasonable
preview of how well shadow and highlight details will show up.

The only problem with this method is that it doesn't mesh well with my
workflow...  you must convert the image to greyscale before the custom
dot-gain curve will work on the displayed image.  And I prefer to work
in RGB (even though it is a monochrome image), until the very last
step before I send the print over to Qimage for cropping, resizing,
and final sharpening.  

So, what I end up having to do is, after converting to greyscale, add
another curves adjustment layer to tweak the almost final image to
achieve the tonal range and contrast that I'm looking for.  Either
that or go back a step and then do some guesswork on the earlier
curves and b/w conversion, then reconvert to b/w and the custom dot
gain curve.

I'm currently satisfied that I can get a good preview of my print, but
would be much happier if I could get the good preview earlier in the
workflow.  If only you could have a good proof-visualization layer in
photoshop... a layer that would non-destructively convert the
displayed image, but would still allow you to work on the underlying
unconverted rgb layers.

Does anybody have any alternatives for this approach?  Where you can
see, on-screen, a fair representation of the final b/w print, but
without having to do the greyscale conversion until you are satisfied
and ready to send the image out for final print preparation.