I am wondering whether this led to any working solutions. I have a mostly identical situation and haven't found a good solution yet. Let me try and clarify ax to the why of this particular configuration.
In my case I have a Mac connected to "the Internet" (obviously via my home router) through it's Wifi interface. My DLNA server (a Synology NAS) is connected directly to the same Mac using a wired Ethernet connection.
Why not connect my DLNA server to the router? Wiring constraints! Yes I could solve these, but for now I would like to find it if it is at all possible to get this setup working.
I've created a bridge and connected both my wireless and wired interfaces to it. That's the easy part. However, as you already pointed out this creates a somewhat unclear situation: two different subnets are now connected to a bridge and because they are different subnets they still can't talk to each other even though they are "connected". At the IP level, things aren't as they should: getting from one network to the other requires a (default) gateway in the respective networks.
The problem is that different networks are now connected at L2, and this shouldn't be done. One resolution might be to make sure that both interfaces are on the same network, but this creates a new problem: two of the Mac's interfaces are then on the same network and this causes an unclear routing decision for traffic sent from the Mac, since both interfaces present a default gateway for traffic outside these networks. To solve this, something has to be done to make sure that one of these interfaces (in this example the Wifi) is always used as the default. Furtermore, traffic destined for the DLNA server must always be routed to the wired Ethernet interface. I have not found a working solution for this yet.
You point out that giving the bridge itself an IP address on the "Internet facing" network should do the trick, without having to put both interfaces on the same subnet. I am curious to find out how this would work. Assuming that both interfaces remain, the only thing that changes is that they are bridged together with a logical bridge that has it's own address. How precisely would that alter OS X's network behaviour?