Being on vacation I gave a shot on building and launching qemu under Windows. Of course I was stumbled on getting the message
chardev: backend "stdio" not found
qemu: could not open serial device 'mon:stdio': No such file or directory
when running with -nographic option. The option is necessary for launching qemu with SPARCstation-5's OBP, as it doesn't support qemu's TCX graphic card.
I googled, but found only that many people have stumbled over this problem before with no luck of solving it. And then I was so desperate that I read the qemu manual page. And you know what? The solution is described there! Instead of using console for standard I/O, it's possible to redirect it to a port, and telnet there:
sparc-softmmu/qemu-system-sparc -nographic -serial mon:telnet::4444,server,nowait -bios ../sparc/ss5.bin -hdd ../sparc/solaris-disk
And then just telnet to localhost:4444.
Now I have Solaris/sparc running on my Windows laptop. ;-)
Moreover I think the option -serial mon:telnet::4444,server,nowait is pretty useful for Unix hosts too. When stated with it you have ability to get into qemu console by pressing Ctrl-A c, and hence eject/insert virtual CD-ROMs, plugging unplugging disks and so on.
Update: the above method works if qemu is compiled under cygwin. With plain msys/MinGW, it has to be started with
-monitor file:bob.txt -bios ..\ss5.bin -m 256 -M SS-5 -serial mon:telnet:127.0.0.1:4444,server,nowait
options. Thanks, Neozeed.