If you have a self-contained repro project for this, I'd be happy to take a look.
Have you enabled Streaming (click the button in Fiddler's toolbar)? If not, a poorly-written client framework could deadlock.
Other than that, the most likely issue with CouchBase having problems with proxies is that the client is failing to properly clean up resources and thus hits a connection pool (or similar) limit and blocks waiting for a connection that never becomes free. There's nothing that Fiddler or another proxy could do to fix this.
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