I've researched this extensively without a clear conclusion, so time
to reach out to the people who have probably experienced it.

I have a plywood subfloor that has several uneven parts. My plan is
to (1) level the worst spots with self-leveling cement; (2) add
another half inch layer of flooring plywood glued and nailed to the
joists. Then I will felt the remaining uneven spots as necessary, and
staple hardwood flooring.

The self-leveling products do not lend themselves well to plywood I
know, and it certainly doesn't seem recommended to add hardwood
directly over SLC. But I'm hoping that my "sandwich" approach will
work. I have previously dealt with such problems by shimming and
using varying layers of felt. But this hasn't always worked very well
- it's very difficult to deal with differences of more than a 16th or
so. In this instance I am looking at some differences as great as
1/2". I should add that the uneveness was caused by a previous
structural issue that has been fixed but that left a residual gap or
two.

Any thoughts gratefully accepted.

Cub.