The problem is that HD coverage is limited, as you say. But you also mention that the HD would be usable in the City, but not the suburbs. But since most HD installs are in cars, and the historically documented usage radio in cars the City and the Boroughs is very very low, there is not going to be much of an audience because there just are not many home and workplace HD radios.
If that audience will not donate to the station, why would they spend money for an HD radio, even if they could find one.
You said nobody donates and yet the station has survived on donations for many years. I don't know the entire history but I hear them say 50 years on the air. This is where good programming comes into play.
If listeners like the program enough, they will do anything they can to hear it. Just like with cable TV and pay radio. Programs have to be really good though. That's up to the station management and the listeners. The initial costs for the IBOC transmitter and equipment could be a problem and as I have stated many times, I dislike IBOC anyway. If IBOC would serve any purpose though, this could be one example. Many stations already use their IBOC channels for less popular programming, maybe not with good results but the WBAI format is rather unique. Again, the station has to decide where to go on this, return to the past and shrink or change and try to reach out to more listeners.