Where access from underneath isn't a thing, your options are limited to say the least. I helped someone in just such a situation and ["P" word warning!] used pocket holes and beefy screws to good effect. The stairs were to be fully carpeted meaning large unsightly holes weren't a problem. That said, even filled and sanded, you'd still see them on painted stairs where I'd try and find another way.
After drilling some small exploratory holes to see what was / wasn't behind the treads and risers, I used the small Kregg jig to make the pockets. You can use any bit slightly larger than the screw head... and a modicum of care... to drill the pockets.
It certainly wasn't the best way of doing things but time and options were limited. That's my excuse anyway. FWIW, it's still good after 2+ years.