I was wondering if stainless steel is an appropriate metal for hilt parts? I know most enthusiasts prefer mild steel, but would stainless steel fittings be safe for use in hema?

Albion seems to use stainless steel in their Maestro line swords, and they have been working pretty well for me so far.

From the Marozzo description:
The sword features rounded edges, stainless steel hilt components, and a treated cord-wrapped grip.

Mechanically and technically speaking, it's quite appropriate. Visually speaking, it's a different order of business since stainless, mild, and carbon steel have subtly different "sheen" when polished. If I'm not mistaken, carbon and mild steel tends to come out more grey while stainless tends to be whiter. Mind that this doesn't always have to be a bad thing; if you understand polishing and have at least some vague idea of how you might use the difference to good effect, it might improve the looks of the sword instead.