Can mod_proxy_html support (charset XYZ) as input?
In version 2, that depends entirely on libxml2, and your charset is supported if and only if libxml2 supports it. In Version 3.1, charset support is much expanded provided mod_xml2enc is enabled. It is normally sufficient just to load mod_xml2enc: it will be configured automatically if you configure mod_proxy_html using ProxyHTMLEnable. In a few cases, you may need to customise charset support further using mod_xml2enc’s directives. Note that some servers send inconsistent and even conflicting charset information, and may generate unexpected results. Setting ProxyHTMLMeta On may help resolve such cases, and will help diagnose problems with extra debug information in the error log.
