I’m trying to cast a string value to a collation name without success. My suspicion is this isn’t possible based upon the error message.
You cannot cast between a token that interpreted as a literal and one that is interpreted as a name. For example: SELECT * FROM 'table_name'::???; you need to supply an identifier in the FROM clause, not a literal.
Identifiers cannot be parameterized, so if you want to interpolate you need to do so while building a dynamic query string. See the "format()" function and "EXECUTE" command for ways to do this within the server. Or use whatever client-side facilities you have at your disposal.