Dynamic specification of column names (and table names) is a feature one is used to when coding ABAP OpenSQL but it is not a feature of HANA SQL-Script.
Note that AMDPs are simply database procedures stored in ABAP methods. Consequently, AMDP simply reflect HANA capabilities.
There is some kind of dynamic SQL in HANA using 'EXEC'-statement and string-manipulation, e.g.:
METHOD amdp_modify
BY DATABASE PROCEDURE FOR HDB LANGUAGE SQLSCRIPT
USING tadir.
declare column_name string := 'OBJECT';
select "OBJECT" from tadir;
EXEC( 'select "' || :column_name || '" from tadir' );
ENDMETHOD.
But then your AMDP can no longer have the READ-ONLY option and it is also difficult to access the result set of the EXEC-statement (when a 'select' is contained in its argument).
In the example above you would probably prefer to do your string-manipulation in ABAP and then use ADBC (http://scn.sap.com/message/16171819).
Dynamic ABAP Open-SQL is just a built-in feature of this string-manipulation (with a limited set of capabilities).