So far as I can tell the only options are:
1.  Update the value after updateDocuments is called
2.  Analyze the field twice, once during the mapper step and once during updateDocuments
3.  Analyze the field during that mapper step and record the tokens for playback during updateDocuments

Number 1 seemed more risky but I figured if I could get it working without any blowing up then it'd be worth it.  Am I being naive to think this could be safe with sufficient testing?

Number 2 seems like it'd be slow especially for reasonably large documents.  Number 3 feels like it'd just swap memory for cpu but be similarly bad.