The normal JSON encoding for Decimal (5.4.3) states to encode just the content of the decimal (Scale and Value), thus to omit the fields which make it look similar to an ExtensionObject (TypeId and Encoding).

The general rules for Decimal (5.1.8) state that when a Decimal is stored in a Variant, then the Variant TypeId should be set to ExtensionObject.

If both rules shall be followed strictly, then it is difficult to decode a Decimal from a Variant for JSON as the common identifier fields are missing.
Applications which don't support the DecimalType won't be able to treat it like an unknown ExtensionObject.

When used in a Variant, then I propose to store the Decimal content as the Body field of an ExtensionObject and use similar rules as for binary encoding.

• Use the Decimal data type ID (50) in the TypeId field
• Fill the Encoding field according to the allowed settings for JSON ExtensionObject (5.4.2.16) (0, 1 or 2 for Structure, ByteString or XmlElement body)
  or only option 0
• fill the Body field according to the EncodingField