I would like to suggest adding "pseudo-fields" that one can map Xpaths too. These wouldn't need to actually be present on the target bundle, but would be used to attach metadata to the entity, that could be used during import.
My use case:
I gather data with the feeds module and xpathparser.
Sample data 1:
<xml> <id>1234</id> </xml>
Sample data 2:
<xml> <id>1235</id> <ref-id>1234</ref-id> <action>delete</action> </xml>
In the incoming XML data, there are two fields, used to signify an old entry should be deleted. In my feeds Parser->parse(), I have
$this->config['sources']['refid'] = 'XML/ref-id';
$this->config['sources']['action'] = 'XML/action';
which makes the parser get these two data values even if I don't map them to a field. But I can find no way of making this information available to the Processor. I would like to find some way of sticking this data on to the $node, so that in entitySave() I can ignore saving id 1235 and instead delete 1234.
Currently I solve this by abusing drupal_static to pass data between the parse and entitySave functions.