To override the default JSON export behavior for specific AEM components, a developer should create a Sling Model that implements the ComponentExporter interface. This model must be adaptable from the component's SlingHttpServletRequest or Resource. When the AEM SPA framework serializes the component model, it checks for a custom ComponentExporter implementation associated with the component's resource type. If found, it uses this custom implementation to generate the JSON output. For components without a custom ComponentExporter, AEM falls back to the default serialization mechanism, thus meeting the requirement to only override the exporter for a subset of components.

A. Implement ImplementationPicker service: This service is used to select a specific component implementation (script) to render based on request parameters, not to customize the JSON export model.
C. Implement FallbackComponentImpl service: This is not a standard, documented AEM service or interface related to the Sling Model Exporter framework or SPA component serialization.

1. Adobe Experience Manager as a Cloud Service Documentation, "Sling Model Exporter": This document explains the core framework. It states, "The Sling Model Exporter relies on the @JsonExport annotation on the Sling Model class... The framework also provides a way to use a custom serializer... A Sling Model can also implement the ComponentExporter interface to get full control over the JSON output."

Source: Adobe Experience Manager Documentation > Developing > Developing with CRX > Sling Model Exporter.

2. Adobe Experience Manager as a Cloud Service Documentation, "Customizing the JSON Output": This guide for SPA development explicitly details the method. It notes, "To take full control over the JSON serialization of a component, the component's model can implement the ComponentExporter interface... By implementing the ComponentExporter interface... the component is given full control over its JSON representation."

Source: Adobe Experience Manager Documentation > Developing > Headless and SPA > SPA Editor > Developing SPAs for AEM > Customizing the JSON Output.

3. Javadoc for com.adobe.cq.export.json.ComponentExporter: The official interface documentation defines its purpose: "Interface to be implemented by a Sling Model that wants to define its own JSON export. The Sling Model must be adaptable from a SlingHttpServletRequest or a Resource." This confirms it is the designated mechanism for custom component-level JSON serialization.

Source: Adobe Experience Manager 6.5.0 Javadoc, com.adobe.cq.export.json package summary.