"brew install rtorrent" now failing to build
Oh, I did manage to get this working. Thought I'd answer my own question. The key was to create a symlink for the missing libisl.10.dylib so that gcc 4.9.2 runs correctly, then recompile libtorrent, then reinstall rtorrent.

Step 1: Downgrade gcc to 4.9.2.

brew switch gcc 4.9.2


Step 2: Ensure the symbolic link for libisl.10.dylib exists.

cd /usr/local/lib
ln -s libisl.dylib libisl.10.dylib


Step 3: Uninstall and reinstall libtorrent.

brew uninstall libtorrent
brew install libtorrent


Step 4: Reinstall rtorrent.

brew install rtorrent


Step 5: To be safe, switch gcc back to the latest version.

brew switch gcc 5.1.0