They make neoprene shorts with large pockets that go over wet suits. I have never seen a fanny pack used as a carry all on any dive I have done - so you may be setting a trend.
It could work but I have pockets in my drysuit and a bungee that I use double enders to attach my SMB, Reel, and things. So my caution is if you open the fanny pack - everything may dump out of it and then you are wasting time gathering them back up or diving down to get those things you dropped. If you try it - have something like a bungee to keep it secure until you need it...