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Trying to change how preprocessing works turned out to be a dead end. While trying to work out the bugs it introduced, I discovered that some commands depend upon it working the old way. But I also discovered an undocumented command that will do what you want. The command is eval. It will evaluate the variable that comes after it as though it were a line of code. Here is some code that lets you use information from the ray operator to shift pieces:

set rowe join 'shift ' list ray a1 1 0;
echo #rowe;
eval #rowe;

The first line creates the line of code for shifting pieces and copies it to the variable rowe. The second line shows you what that line of code looks like. The third line executes the line of code.