我相信我已经解决了:

<cfheader name="Content-Length" value="#getPageContext().getCFOutput().getBuffer().size()#">

我把它卡在 onRequestEnd() 中，Cisco 盒子很高兴地压缩了.

I stuck that in onRequestEnd() and the Cisco box is happily compressing away.

感谢大家的投入.