For those of you also seeking such a solution, I just wanted to note that I unsuccessfully tried the following:
1. Prefixing the ASIN with "asin", "asin_", and "asin|"
2. Using the Roku Master Content ID (or Play ID) which is the Gracenote TMS ID - with and without the prefix ("MV" or "SH")
3. Using the Gracenote TMS Root ID
4. Including source=meta-search
It seems unlikely that Amazon would have created a new numbering system for this, and since the functionality must exist because Amazon is supported in the Roku Search and Ad Link functionalities, I am hoping a Roku Tech has the answer for me 🙂