USB CDC ACM device cannot be set to use 1.5Mbaud
It is a driver problem. The com.apple.driver.usb.cdc.acm driver actually instantiates an IOSerialBSDClient for the device, as seen in the output from ioreg:

+-o IOUSBHostInterface@1  &lt;class IORegistryEntry:IOService:IOUSBNub:IOUSBInterface, id 0x100007a62, registered, matched, active, busy 0 (208 ms), retain 7&gt;
  +-o AppleUSBACMData  &lt;class IORegistryEntry:IOService:IOSerialStreamSync:IORS232SerialStreamSync:AppleUSBSerial:AppleUSBACMData, id 0x100007a6c, registered, matched, active, busy 0 (1 ms), retain 6&gt;
    +-o IOSerialBSDClient  &lt;class IORegistryEntry:IOService:IOSerialBSDClient, id 0x100007a72, registered, matched, active, busy 0 (0 ms), retain 5&gt;


Which is very limited in the number of baud rates that it supports. Version 11 of the IOSerialFamily, the version on my machine, only supports the baud rates listed in the static struct speedtab iossspeeds[] in the IOSerialBSDClient class source. I tested this by successfully setting the baud rate for the interface to 1843200.

The solution, unfortunately, will probably require me to locate a generic driver that supports the baud rate I require.