I do something like what you want and it works fine for me. I have two areas that I use BitTorrent sync to keep up to date between a desktop, laptop, and another backup
The first directory is LRWork. It contains various subdirectories which contain photos I am working on (recent shoots/trips). It also contains Catalogs/ and there I have my master catalog, previews, and smart previews. This directory is kept on my laptops internal hard drive so its always available.
The second directory is Photos which contains all my archived photos that I am "done" with. Most of those I have generated smart previews of which allows me to work on them if needed, but not generate a JPG sufficient for a print. That is kept on an external drive for the laptop and is only fully available when connected.
BitTorrent sync happily keeps all of this up to date across machines. The only key is that you have to remember to pause syncing when you start Lightroom. If you don't do this, its possible to get a corrupted Catalog. Should that happen you should have backups and BTSync itself will keep old versions. It's happened to me once or twice and I was able to recover with minimal losses.