I've been using OpenWRT/MLPPP (fw5's firmware) on a Netger WNDR3700... It works, more or less, but rarely manages to connect both lines on startup or after one or both lines go down briefly. Almost any change in connection status forces you to manually tell the router to disconnect and reconnect the bundle.
I expect they'll fix that at some point, but it's been more than two years since the latest update, so I can't really recommend OpenWRT/MLPPP at this point unless you understand those limitations. I don't know enough about Mikrotik's implementation to say if it's a viable option or not (hopefully it is, but I doubt they automatically configure the various MTU/MSS/MRRU/etc stuff), and ZeroShell/MLPPP hasn't been updated in ages.--
Developer: Tomato/MLPPP, Linux/MLPPP, etc »fixppp.org