Yes, redaction has been requested previously, will be good to have.
Further developing on AdamS idea, there should be two separate features with similar interface: highliting and redacting.
So for workflow:
Do some kw search with optional facet filtering, select all (or some documents), right click ->higlight results.
this will create the PDFs linked to originals with say 50% transparent yellow and add them to the (new) special property "highlighted" selectable from the facet list.
Clear all kw searches, lists, etc. (required!), select some documents from the highlighted facet, right click -> redact the highlited parts
that will change the 50% yellow to 100% black (asuming B/W documents, it will not work on colorfull html), then cut the text, replace it with say * (including spaces) and save the PDF, add to the new special property "redacted".
Add "Export only redacted data" checkbox in the appropriate dialogue box or put some other safenet on exporting.