It is possible to run altera_avalon_uart at 921600 bps?

Modifying 'altera_avalon_uart_hw.tcl' is a very clean way of doing it. However, this script does perform a number of checks on your chosen values and won't simply accept anything you throw at it. I've successfully added 250kbaud before. However, adding 921600 didn't work - the script's checks threw it out. This was (most likely to be) due to it not complying with a valid baudrate error the script allows. This is clearly dependent on your source clock frequency. You may find some settings that allow it to work. However, you may have to modify the script further so that the checks allow it to use your chosen baudrate and source clock frequency.