A question on systable_beginscan()

As it's documented in the source code, systable_beginscan() could be used to on non-system tables as well. My question is that, is it possible to write a C code with systable_beginscan(), systable_getnext() and ScanKeys which is equivalent to the following query: (Assume that the qual_column is a text column)

SELECT id FROM table WHERE qual_column::int = 15;

In other words, can we cast a column type to another type via these low-level APIs? Are there any examples in the Postgres source?

Thanks,
Onder