To solve this I ended up using a hyper complex partial to replace all URLs by absolute ones in the .Content
variable on feeds:
{{ $html := htmlUnescape .Content | safeHTML }}
{{ $hrefs := findRE "href=\"([^\"]*)\"" $html }}
{{ range $href := $hrefs}}
{{ $absHref := strings.TrimPrefix "href=\"" $href }}
{{ $absHref = strings.TrimSuffix "\"" $absHref }}
{{ $absHref = printf "href=\"%s\"" ($absHref | absURL) }}
{{ $html = replace $html $href $absHref }}
{{ end }}
{{ $srcs := findRE "src=\"([^\"]*)\"" $html }}
{{ range $src := $srcs}}
{{ $absSrc := strings.TrimPrefix "src=\"" $src }}
{{ $absSrc = strings.TrimSuffix "\"" $absSrc }}
{{ $absSrc = printf "src=\"%s\"" ($absSrc | absURL) }}
{{ $html = replace $html $src $absSrc }}
{{ end }}
{{ $srcset := findRE "srcset=\"([^\"]*)\"" $html }}
{{ range $set := $srcset}}
{{ $parts := strings.TrimPrefix "srcset=\"" $set }}
{{ $parts = strings.TrimSuffix "\"" $parts }}
{{ $parts = split $parts "," }}
{{ $newSrcset := slice }}
{{ range $part := $parts }}
{{ $part = $part | replaceRE "^\\s*(.*)\\s*$" "$1" }}
{{ $lg := split $part " " }}
{{ $href := index $lg 0 | absURL }}
{{ $size := index $lg 1 }}
{{ $newSrcset = $newSrcset | append (printf "%s %s" $href $size) }}
{{ end }}
{{ $newSrcset = delimit $newSrcset ", " }}
{{ $newSrcset = printf "srcset=\"%s\"" $newSrcset }}
{{ $html = replace $html $set $newSrcset }}
{{ end }}
{{ return $html }}