I was wondering if, other than annealing, a means could be found of altering the magnetic properties of ferrous metals, especially nickel.

I found one such means... hydrogenation. Hydrogen has a strong affinity for nickel (and iron and cobalt).

Nickel adsorbs molecular hydrogen (which attaches to the surface) and absorbs atomic hydrogen (which then tunnels into the bulk of the metal), which reduces nickel's magnetic dipole moment. It does this by changing the band structure and density of states of the valence electrons, leading to a decrease in the spin-majority density of state and an increase in the spin-minority density of state.

This reduces the hysteresis of the metal, in effect making it more paramagnetic.

Desorption of the hydrogen occurs for molecular hydrogen at ~350K (170 F) and for atomic hydrogen at ~200K (-100 F).

The tradeoff for a reduced magnetic moment is an increased electrical resistivity of the hydrogenated metal. Depending upon the intended usage, this could be detrimental or beneficial.

Some reading material: