The draft of the revised HTTP/1.1 (should be RFC'd shortly) says other response header can be included in 304 responses if they exists for the purpose of guiding cache updates.
The server generating a 304 response MUST generate any of the
following header fields that would have been sent in a 200 (OK)
response to the same request: Cache-Control, Content-Location, Date,
ETag, Expires, and Vary.
Since the goal of a 304 response is to minimize information transfer
when the recipient already has one or more cached representations, a
sender SHOULD NOT generate representation metadata other than the
above listed fields unless said metadata exists for the purpose of
guiding cache updates (e.g., Last-Modified might be useful if the
response does not have an ETag field).
This issue is to make it possible to include Last-Modified in a cached 304 response.