Can't be too hard to code.
qboolean G_admin_cvarset( gentity_t *ent, int skiparg )
{
char name[ 100 ]; // arbitrary value here since I'm lazy
char value[ MAX_CVAR_VALUE_STRING ];
G_SayArgv( 1 + skiparg, name, sizeof( name ) );
strcpy( value, G_SayConcatArgs( 2 + skiparg ) );
trap_Cvar_Set( name, value );
}
Obviously you'll want to throw in some error checking, but that's the basic idea.