Bit of follow up...
Wasn't able to find a way to make the service publish without a default schema using a single value input parameter. If you use a multi-value input parameter it appears to be less defined.
However, the key to using the service either way is to make sure to include the 'fields' object on the FeatureSet json. When the fields object is included, your schema follows thru as you'd expect. If the fields object is NOT included in the json, then the 'default' schema is used --- which was what caused my original issue.