It is possible to run altera_avalon_uart at 921600 bps?

The limitation comes from the baud ALLOWED_RANGE property in the _hw.tcl of that module, and I believe it is only restricting you when creating a component with fixed baud rate.

There are many ways to work around it: don't use fixed baud rate, clone and modify the Altera supplied copy of the module, or don't do any of the above and just supply it with 8x the clock you told it you would give it.