Aluminium for a plate is pretty unusual, but not unheard of. The BBC Microcomputer largely used aluminium plates:http://wouter.bbcmicro.net/pictures/com ... index.html
The two AWC keyboards (Futaba and SMK switch versions) had aluminium plates, as did the PED keyboard. The only version with a steel plate was the SMK keyboard, and Wouter considers this to be a downgrade from aluminium. Nobody has the faintest clue who PED were, and I don't know whether other AWC keyboards also used aluminium — the only other keyboards I know they made were for the Atari 800 series.
I don't understand why the choice of material would have any effect on the size of the mounting holes, and if the plate is bending because the holes are the wrong size, try making the holes the correct size ;-)