It is not possible to change the internal boot ROM Code.
I will check this internally and update you.
When I tested, i could not notice any toglling on I2S_SD pin. It was low as expected.
- Madhu Sudhan
Same for me, I checked GPIO during reset/power-up and I2S_SD (GPIO51) behaved as specified (drive low).
Please find a list of GPIO values in Knowledge Base Article KBA86731 (http://www.cypress.com/?id=4&rID=91267)
Cordially - Frank