Can I use HAProxy's new 'capture' feature to save the remote address in a TCP frontend, and use it as the `X-Forwarded-For` header in an HTTP backend?

Using HAProxy 1.6 and a clever hack, I now have an HAProxy tcp mode frontend, that detects if the browser is capable of SNI, and based on that, routes to a strongly ciphered SSL termination backend, or a weaker one. This ensures A+ grading on SSL labs, while still allowing all browsers except IE6 to use SSL.

Here is my config. It has some template variables in it that should be self-explanatory, but aren't in areas relevant to my question:

frontend https_incoming
 bind 0.0.0.0:443
 mode tcp
 option tcplog
 tcp-request inspect-delay 5s
 tcp-request content accept if { req.ssl_hello_type 1 }
 use_backend https_strong if { req.ssl_sni -m end .transloadit.com }
 default_backend https_weak

backend https_strong
 mode tcp
 option tcplog
 server https_strong 127.0.0.1:1665

frontend https_strong
 bind 127.0.0.1:1665 ssl crt ${DM_ROOT_DIR}/envs/ssl/haproxy-dh2048.pem no-sslv3 no-tls-tickets ciphers ${strongCiphers}
 mode http
 option httplog
 option httpclose
 option forwardfor if-none except 127.0.0.1
 http-response add-header Strict-Transport-Security max-age=31536000
 reqadd X-Forwarded-Proto:\ https
 reqadd FRONT_END_HTTPS:\ on
 use_backend http_incoming

backend https_weak
 mode tcp
 option tcplog
 server https_weak 127.0.0.1:1667

frontend https_weak
 bind 127.0.0.1:1667 ssl crt ${DM_ROOT_DIR}/envs/ssl/haproxy.pem no-sslv3 ciphers ${weakCiphers}
 mode http
 option httplog
 option httpclose
 option forwardfor if-none except 127.0.0.1
 http-response add-header Strict-Transport-Security max-age=31536000
 reqadd X-Forwarded-Proto:\ https
 reqadd FRONT_END_HTTPS:\ on
 use_backend http_incoming

Problem: the https_incoming frontend knows the Client IP, but since it is in mode tcp, it cannot save this information in a mode http X-Forwarded-For header. option forwardfor is not valid in TCP mode.

From another question on serverfault I already found that I could use:

• LVS
• PROXY protocol

So that the X-Forwarded-For header isn't even needed anymore as from what I understand, in the case of LVS: packets are spoofed so the source becomes the Client IP, and in the case of PROXY: packets are encapsulated to carry the Client IP.

These both seem like they could work. LVS however seems quite a heart-surgery for us that could have side-effects, and PROXY has the downside that proxies/application upstream/downstream, might not be fully compatible yet.

I was really hoping for something more lightweight, and that's when I found the new "Capture" feature of HAProxy 1.6 as it mentions:

you can declare capture slots, store data in it and use it at any time during a session.

it goes on to show the following example:

defaults 
 mode http

frontend f_myapp
 bind :9001
 declare capture request len 32 # id=0 to store Host header
 declare capture request len 64 # id=1 to store User-Agent header
 http-request capture req.hdr(Host) id 0
 http-request capture req.hdr(User-Agent) id 1
 default_backend b_myapp

backend b_myapp
 http-response set-header Your-Host %[capture.req.hdr(0)]
 http-response set-header Your-User-Agent %[capture.req.hdr(1)]
 server s1 10.0.0.3:4444 check

It appears to me, information is stored in a frontend, and then later used in a backend, so perhaps I can take the Client IP in TCP mode, save it, and use that later down the line, maybe like so:

http-response set-header X-Forwarded-For %[capture.req.hdr(0)]

I've looked at the capture docs and there it seems capture is more oriented at http mode headers, but then I have also seen a mailing list conversation successfully demonstrating the use of a tcp-request capture.

I've tried several things, among which:

tcp-request capture req.hdr(RemoteAddr) id 0
# or
tcp-request content capture req.hdr(RemoteHost) id 0

But as you can see, I haven't got a clue what the syntax should be and under which key this information would be available, nor can I find it in the (I think) relevant documentation.

Questions: Would it be possible to capture the Client IP in TCP mode, and later down the line, write this information into the X-Forwarded-For header in HTTP mode? If so, what would be the syntax for this?