It is possible to run altera_avalon_uart at 921600 bps?

For what it's worth, 921600 with a 25MHz or 50MHz clkin commonly used on the eval boards has zero error and no warning from the script.

Should you care for an even more odd-ball baud rate, the only thing you should need to have to do is e.g. use a PLL to supply a clkin as an integral multiple of the baud rate.