Content-Range and Caching Proxies

I am trying to add range support within my proxy following that
scheme: the proxy gets a document, and caches it in a local
file. Latter on a request comes in for that document (which is still
fresh according to both the request and the cache entry) with a
range specification. In that case the proxy will (should/could ?)
handle the range request. 

My concerns are:
- Should the proxy emit an 'Accept-Range' header when serving the
  cached entry (since now that it has entered the cache, the cached
  resource will handle ranges)
- The spec says (13.5.2) that I can implement that only if the
  Cache-Control directive 'no-transform' has not been set by the
  origin server, I am not sure I understand the rationale. In
  particular it seems to me that this case is (potentially) much less
  "harmfull" than if I were to transform gif images to jpegs - I mean
  transform the entity - and the client will not be able to
  distinguish between these two cases since they use the same
  warning). 
  [that same remark applies to the case where you want to use byte
  ranges in the proxy (instead of supporting them) ?]

I guess I miss something, though,

Anselm.