Hi Russ,
This is a cross-post answer from your same question on the Controlled Vocabulary list.
In other words will the thumbnail and preview reflect changes made in ACR?
Unless I'm mis-understanding you no image browser or DAM outside the RAW converter itself will show its edits. This is true for Lightroom, Aperture, Bibble, Adobe Bridge+ACR, Phase One Capture One, Nikon Capture. Photo Mechanic is not alone here - it's just how image browsers work. Bridge is the only browser that will show ACR edits and is partly why it's so DAM slow (pun intended). You could also complain to Adobe that Bridge doesn't show your Capture One edits.
The best workaround is to use DNG and update the embedded preview which PM, iView, etc, use. Unless your RAW converter can update the embedded preview inside the RAW file itself DNG is the only viable option.
Alternatively if you are converting to JPEG have ACR save them in the same folder as your RAW files. That way PM can display the JPEG+RAW as one and give the same affect as Bridge. You can set PM to only open the RAW when you want to edit and you can open multiple RAW files in ACR. One gotcha is you can only send RAW files to ACR c/- Photoshop and not Bridge. Bridge lets you save in background and continue editing on other RAW files. ACR via Photoshop locks you out until all the files have been converted and saved.
FWIW Phase One assure me Capture One 4 will import and export DNG and update the embedded preview. What size that preview will be is not clear. DNG is becoming more appealing.
-- Robert Edwards.