I have a few cities with bus-only bridges, bus-only express lanes, and bypasses. Some of them work really well, some of them work... sort of.

Also, you need NAM for buses to be viable. What is BRT?

This bridge in my reddit co-op city was the first bridge across this river. after I built the avenue bridge to the north of it, I made it bus-only. http://dl.dropbox.com/u/33796991/Simcity/hurricane%20may21/carina%20deneb%20bus%20bridge.png

This worked when most of the city was on this landmass, but since the downtown is across the river, there's hardly any commuters up and down this avenue. I have a road marked for bus-only in the middle of two OWRs. As you can see, now people only take it for a block or two. http://dl.dropbox.com/u/33796991/Simcity/hurricane%20may21/electric%20avenue%20bus%20lanes.png

Here's another bus-only 2-lane bridge which works pretty well. The avenue bridge to the south gets about 2000 to 4000 daily crossings, mainly on car but some buses as well. That's one of the main highways running from the suburbs straight into the downtown core. http://dl.dropbox.com/u/33796991/Simcity/hurricane%20may21/betelgeuse%20bus%20bridge.png

This isn't an HOV-only lane, but it is an elevated viaduct to bypass surface traffic like you described. It runs through an early-20th-century brownstone neighborhood, and the one avenue along the shoreline is nowhere near enough to handle the traffic. There is only one intersection in the middle of it, except for some people who somehow jump off the edge directly into their office or bedroom windows. It was an experiment, but it made a major difference in the traffic congestion in the area. http://dl.dropbox.com/u/33796991/Simcity/hurricane%20may21/orion%20river%20-%20jamison%20st%20viaduct.png

As for the busway project you described, I would only use that with NAM anyway because of the elevated roads. Unless your city is mostly high density you probably wouldn't need multiple lanes - buses are pretty efficient when everyone is going to the same place. That brings me to the second issue - I'd only build it if your zoning is very divided so everyone is going in herds to the same areas. Most of the time that I've wanted to do something like that, the commuter traffic was too spread out randomly around the city for it to make much difference. But if your city is separated by a river, a bus-only bridge can be really helpful.