I wouldn't use a stainless screw, I don't care how much anti-seize you put on it.
Stainless steel + aluminum + salt water = a battery
Galvanic corrosion is the result.
And the aluminum is the less noble metal. New rudder castings are $$$$
On my 14 restoration project, I'm dealing with a fool that replaced his aluminum rudder pins with a steel bolt. To top it off, he drilled and tapped for a steel set screw in the casting to hold the bolt in. Both the bolt and the set screw are now completely frozen. I had to saw through the bolt in several places just to get the castings off the boat.
In contrast, the threaded nylon plugs came out without having to resort to the "heated screwdriver" trick.
There's a reason the plugs are inert plastic.