intertwingly

In processing a comment API request, I may end up grabbing excerpts, formatting, etc.  I can also spell check.  Would there be interest in a variant of the comment API which simply returns back what would have been posted instead of actually doing it?

It seems to me that there are the following options for implementing this API:

• Add an optional element within the RSS item itself indicating that a preview is desired.
• Add an element outside the RSS item but inside the HTTP body.
• Add a HTTP header.
• Change the resource (i.e., URI) to which such requests are posted.

One thing I think is important is that such an option not be ignored.  In other words, I presume that a client would prefer a failure (HTTP status and/or SOAP fault) than to have the request actually processed.  Unless somebody sees something I don't, that reduces the options to two: SOAP headers with mustUnderstand attributes have this semantic, and clearly routing to a different URL can be presumed to do this effectively.

Suggestions?