I've trialled the other feeds+entityreference things and they didn't work for our use-case, so I abstracted the actual task, added some smarts and produced
feeds_tamper_string2id today.

Huge description over there.

* get a label from a feeds data source
* tamper it (eg explode or convert case)
* tamper it to convert the string(s) into entity IDs by making lookups on our own data (existing node titles, views field values)
* store entity IDs as entityreference
= auto-linked imported data.

Just FYI.