I wonder if anyone has tried this. The only other way I know of to soft-proof if one is using LR is to go to Photoshop for printing (what I actually do). For those who need Photoshop for no other reason and therefore don't want to buy it, and have a Mac, I'm wondering if there's another way.
The workflow I have in mind:
1. Get the image ready to print in the Develop module.
2. Export it to a JPEG with AdobeRGB or ProPhoto RGB color space.
3. Open the image in ColorSync and use the drop-down at the bottom to apply the printer's profile (e.g., SPR1800 PrmGlsy BstPhoto.icc). The image will change (probably lose tonal range).
4. Back in LR, increase the brightness, or adjust the curve, or whatever.
5. Repeat from Step 2, but this time compare the ColorSync soft-proof to the LR image after using the history to temporarily back out the adjustments in Step 4.
6. When the soft-proofed image looks right, print it from LR.
Yes, this is certainly awkward, not nearly as convenient as soft-proofing in Photoshop, but maybe it's actually workable. It's certainly easier, faster, and cheaper than hard-proofing (printing and then making adjustments based on the trial print).
--Marc