STIX Custom Properties specs

From the specs (2.0 and 2.1)

"A STIX Object MAY have any number of Custom Properties."

for example:

{

  ...,

  "x_acme_org_confidence": 10,

  "x_acme_org_scoring": {

    "impact": "high",

    "probability": "low"

  },

  ...

}

I've been thinking of implementing this in my scala library at:

      [scalastix](https://github.com/workingDog/scalastix)

however it is turning into a mess to have unknown key names.

I would prefer to have a dedicated area/object, say "x_custom", such as:

{

  ...,

  "x_custom": {

      "x_acme_org_confidence": 10,

      "x_acme_org_scoring": {

        "impact": "high",

        "probability": "low"

      },

    ...

  }

}

Would this scheme of putting all custom properties into such "object" be a break from the specs?