Threaded rod through the treads would be nice but drilling a 4' long
hole through the 3" thick treads would be tricky.

Why not mortise the treads in then use two threaded rods going across
between stringers just below each tread?? [...]
If the treaded rods appearance bothered you they could be hidden by
a square piece of 1.5 by 1.5 rabbeted out to capture the rod and
enclose it under the tread.

Another way to do this is to cut a dado along the length of your
tread the width and depth of your threaded rod, that way the rod
is hidden from the front without an additional piece, and it's a
lot easier than drilling. Centered on the bottom face if you're
using one, an inch or two in from each edge if you're using two,
say.

Or if you're going to eventually box these stairs in, you could
just put the threaded rod far enough back that it gets hidden by
the riser...
--Glenn Lyford