Are bwa alt contigs controlled purely by idxbase.alt?

Question

[d-cameron]

What is the source of bwa ALT alignment? The documentation indicates that:

bwa reads the ALT contig names from "idxbase.alt"

Does this means that the index itself contains no information about ALT contigs? Would I be able to JIT assign a different set of ALT contigs for different samples without having to rebuild to the index?