It is possible to export field_bases separately from field_instances.
It is possible to import these field_bases and they will show in the features table.

Expected behaviour:
When managing fields, they should be available to be added as "existing fields".

Actual behaviour:
The fields can not be added via "manage fields". They do not show up as "existing fields" and recreating them using the same machine name will provoke an error (name already in use).

I am unsure if this is

a) a features bug
b) a field api bug
or c) I'm holding it wrong

thus, filing this as a support request.
Feedback appreciated.