I have done a lot of shore diving from Grand Cayman but I have not been to Cayman Brac. All the sites are marked with a white bouy in the Caymans - if you can see a bouy and swim to it, you are set. There are a lot of good shore dives, some with easy access. The best shore dives were a bit of a swim but worth it - surface swim to the bouy drop down on the wall, turn around at 1500 psi and start your assent, your swim to shore served as a great decompression/safety stop. This was a lot better than the cattle boats, you are not limited to 100 ft max, more bottom time, you can dive north side sites rarely done by the boats.
For Grand Cayman this a great website for shore diving.
http://caymanislandsdiscounts.com/offshore_diving.htm
Not so good for Cayman but great for other shore diving info. I dove several of the sites on the Big Island of Hawaii
http://www.shorediving.com/
The best dive book for Cayman diving is "The Dive Sites of The Cayman Islands"
Here is what they have listed for shore dive sites
North Shore
1) Jan's Reef & Shark Hole (difficult from shore, 25-50ft)
2) Greenhouse Reef (difficult from shore, 25-50 ft)
3) Plymouth Reef (difficult form shore 30-50 ft)
4) Radar Reef (difficult from shore, 30-50 ft)
5) Snapper Reef (difficult from shore, 30-50 ft)
6) Grunt Valley (30-50 ft)
7) Patch Reef (30-50 ft)
8) Charlie's Reef (30-50 ft)
9) Kissimmee Rock (100m swim, 45-65 ft)
10) Fisheries Reef (long swim, 15-50 ft)
South Shore
1) Pillar Coral Reef (200 m swim, 30-50 ft)
2) Cayman Keys & Heddy's Reef (200m swim, 30-50ft)
3) Fry Cave (200 m swim, 30-50 ft)
4) Prince Frederick Wreck (20-50 ft)
Unfortunately the book does not provide info on where to park and tips on the entry point. But you don't have to worry about missing the reef since you just swim to the bouy.
I'm courious to here what you think of the shore diving on Cayman Brac. Send me a PM when you get back and let me know what sites are worth checking out.