Setting "name":"value" as an additional field in the signup express endpoint /admin/api/signup.xml) via the Account Management API overwrites the "org_name" parameter.
Below is a sample request where I've set the org_name to "cake" but passed name=test
curl -v -X POST "https://am1-admin.3scale.net/admin/api/signup.xml" -d 'access_token=<access_token>&org_name=cake&username=cake2%40alanmoran.me&email=cake2%40alanmoran.me&password=password1&account_plan_id=2357355893863&service_plan_id=2357355942110&application_plan_id=2357355948384&name=test'
Below is the trimmed response. You can see org_name is set to test.
<?xml version="1.0" encoding="UTF-8"?> <account> <id>2445582540617</id> <created_at>2018-05-07T01:18:54Z</created_at> <updated_at>2018-05-07T01:18:55Z</updated_at> <state>approved</state> <org_name>test</org_name> ............................................ </account>