Amazon S3 for Stage / Production only?
I made a similar patch to the Amazon S3 module: &quot;New variable to override file field URI scheme per environment&quot;  https://www.drupal.org/node/2422977#comment-9602981
This patch was adopted by the module, and is now the approved way of solving the OP's problem using the Drupal 7 Amazon S3 module. This is the issue I wrote on Drupal.org.

Problem/Motivation
When using Features (or hook_field_default_field_bases() directly) to export file field definitions, the Upload destination is included. This can already be handled in a custom module by implementing hook_field_default_field_bases_alter(), and checking $conf (set in env-specific settings files, etc). But it would be nice to include support directly in amazons3.module.
Proposed resolution
Implement hook_field_default_field_bases_alter(), and check for a namespaced variable amazons3_file_uri_scheme_override. This variable can be added to environment-specific sites/*/settings*.php files to allow certain environments to upload to S3 while other environments upload to whatever the defined/exported (public or private) URI scheme is.
User interface changes
The only change in the UI would be that the field settings reflect the URI scheme defined either by the exported field bases or the variable (on environments where that's present).
API changes
No API changes, but the patch documents usage of the variable.

Note the module's README.md also now contains this additional line:

When using Features to export field definitions, the Upload destination is included. If you want to override this (for example, in a multi-environment workflow), use the 'amazons3_file_uri_scheme_override' variable. See amazons3_field_default_field_bases_alter() for documentation.

Here is the relevant documentation of amazons3_field_default_field_bases_alter that README note is referring to:
* Allows a variable to override all exported field bases to use 'Amazon S3' as
* the Upload destination. For example this can be added to environment-specific
* Drupal settings files, to allow certain environments to upload to S3 while
* other environments upload to the exported (public or private) URI scheme:
* @code
* $conf['amazons3_file_uri_scheme_override'] = 's3';
* @endcode

Also note the variable is not deleted by amazons3_uninstall() because it's only valid use case is being defined in $conf (otherwise just change the field setting).