That's a good question and thanks for posting it to the forum. I'm currently working to find out if there's any additional documentation available, of course, other than what's publicly available. If I find out that there is, I'll reach out to you and let you know.
Thanks for your interest in the Visa Direct mVisa SDK. To move forward with the next step, it's important for you to review the Visa Direct documentation via this link - https://developer.visa.com/capabilities/visa_direct/docs
In particular, please see the top few paragraphs that are shaded in yellow that discuss the requirements to use Visa Direct.
After you've read the documentation, if you meet those requirements, then please click through the link in those paragraphs to the Visa Direct Program Implementation Questionnaire, fill it out, and submit to the email address listed on that form.
Someone will reach out to you soon after to discuss your request.