I had a quick look today at the sq-aflash.bin. For the "BIOS ROM checksum error" message, there is no code path that does not print it.
For the floppy drive error, this sequence: outb 0x3f2, 8 delay outb 0x3f2, 0c (i.e. the act of bit 2 moving from 0 to 1) should raise irq 6 (int 0x0e) after some (~250ms) delay.
That will cause the flow to reach the floppy read. It needs a floppy disk (image) to be present and readable. The BIOS does not support reading from a hard disk, so the CHD cannot be booted.
However, I don't know how to implement the port 0x3f2 behaviour, nor how to provide a floppy image in order to test it.