I tried DD-WRT. It seems to have issues of its own, so it's not a solution for my use.
DD-WRT would not stream live TV from my HDHomeRun tuner for more than a few seconds before dropping. It wouldn't even do it across wired connections. This may have to do with DNLA multicast or UPnP support. I turned on UPnP, which is off by default. I didn't see any DD-WRT options for multicast, so I gave up on it.
The ASUS bridge also often had trouble even seeing the DIR-878 on 5Ghz. This seemed to vary with channel and channel width settings.
I tried both r34311 and r34578. Same streaming issue on both.
Once connected, DD-WRT otherwise seemed OK. File copies and surfing were good. I did some streaming of recorded video, which was was good for the time that I did it. I didn't see the same dropouts as with the stock firmware, but I didn't test it for a long period.
Flashing back and forth was easy. Thanks for the instructions.