And I wonder if I could do this by adding additional inductance in
series with the horizontal deflection yoke, with the idea that more
inductance would slow down the rate of change in the yoke, so that
the beams don't progress horizontally as fast as before, and
therefore finish the line before reaching the edge of the screen.




Usually that's precisely how it's done. Look for an inductor in the
horizontal section with an adjustable slug, it may have enough range to do
what you want without even modifying anything.