I really like the idea as I'm running something similar but I don't know how well a separate system will work for filtration purpose.
The reason for water changes is two fold, first to remove contaminants but we also do water changes to replace major and minor elements depleted out of the water. Placing waste water in a mangrove tank will take care of a large portion of the contaminants but if anything it will actually deplete those essential elements even more.
If you would like to increase the time between water changes what about plumbing your mangrove tank right into your system? This way the mangroves are supplied with a fresh supply of waste to process. Another problem you might run into with a separate system is that the water might actually become an anoxic, this could result in the production of Hydrogen sulfide and adding this water back into the tank could cause some problems.
This tank is plumbed into my 350g system, currently has about 150 mangroves in it and man do they grow fast!
I'm also running what I consider a "Cheato River", which is pretty much your exact design just a little longer with a bulkhead to tie it into the system. When I have frags they go in here, otherwise it's full of cheato.
