Hi Colin,
I would not recommend trying to customize simulated step returns. I do not believe there is an existing library for this purpose, and it would be complex. That said, as long as you have sample tracking enabled then there are easy programmatic ways around this.
Linked below is a post that outlines various examples of barcode assignment and retrieval with minimal code changes needed to accommodate simulated runs.
@Stefan - the image URLs in the linked are from the old domain and I cannot edit the post due to its age. Would you mind restoring them if you have a quick second?
Thanks.
-Nick