i've never used stop+a on an intel machine... on sparc you go to "ok" prompt. intel based machines don't have an obp, so where do you expect to go with stop+a?
I'm afraid even an hypothetical Stop-A equivalent isn't going to help assuming your goal is to go into the boot-monitor.
The boot prom command line interpreter is SPARC specific and has no equivalent with x86 hardware.
Bro... sorry I forgot that you're using x86. For x86, NO ok prompt. you have to connect keyboard and mouse. power up your system and it'll bring you to the "black screen" (BIOS) and you have to press F1 or something.. I can't remember. you can change whatever settings in there or config ur RAID etc.
The closest equivalent in Solaris 10 to the ok prompt is GRUB or bios depending on what you want to achieve. If you want to give additional kernel parameters then it would be the GRUB menu however if you want to control boot devices that would be the BOIS screen for your system