I'm talking about devices like the ones on this B/h page.
Multimedia Hard Drives | B&H Photo Video (http://www.bhphotovideo.com/c/shop/10644/Computer_Media_Centers_Extenders_Multimedia_Drives.html)

I'm a Mac without a viable HD playback strategy.

My question: Are these devices viable solutions?
1) Capture ProRes 422
2) Edit in ProRes
3) Export timeline as is
4) Compress in compressor to some usable format that can be played back on one of these multi-media extenders.

I think I've seen some things where the files are limited to 4gb. I don't know if this would be a problem.

In the end, I'm looking for an HD playback solution that doesn't involve Blue Ray or connecting a camera to a TV.

I can't comment on any of those specific models, but in generic terms yes they should do what you want. Most should play HD wmv, mpgeg2 and m2t... some will also do mp4 / H.264. The cheaper ones may top out at 720p/1080i, the better ones will do 1080p.

Some use pata and some use sata drives: either way make sure they can be formatted as NTFS rather than FAT32 to avoid the 4GB filesize limit.

Personally I can vouch for the awesomeness of the TViX M-4100SH , although it is a bit pricy (~$350).

When comparison shopping, having checklist similar to this might be helpful: Tvixbox.com - unparalleled multimedia players! (http://www.tvixbox.com/comparison.php)