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?

To answer my own question, this does not seem possible, as the traffic 'leaves' HAProxy here:

       TCP                             HTTP
frontend->backend (->leaving->) frontend->backend

So the context is lost and the capture cannot be preserved. Instead, as "PiBa-NL" suggested on IRC at #haproxy on Freenode yesterday:

[5:29pm] PiBa-NL: kvz, use proxy-protocol between back and front
[5:54pm] kvz: PiBa-NL: Thanks, does this mean my app also needs to understand 
         the proxy protocol, or will it be 'stripped' once it reaches the 
         backend. I don't think my node.js server could handle it without  
         significant changes to its stack
[6:07pm] kvz: Or let me rephrase: could I enable the proxy protocol on the first 
         frontend, then 'unwrap' it in the second frontend, taking the client ip 
         and putting it into the http header - so that my app would not have to 
         be proxy protocol compatible, and it would just be means to carry the 
         client ip from first frontend to the second?
[6:49pm] PiBa-NL: kvz, the last part you can still use the x-forwarded-for header
[6:50pm] PiBa-NL: but between haproxy backend and frontend you would use the 
         proxyprotocol to make the second frontent 'know' the original client ip
[6:50pm] PiBa-NL: server https_strong send-proxy
[6:50pm] PiBa-NL: bind ssl crt .... accept-proxy
[6:52pm] PiBa-NL: the second frontend can then still use the  
         'option forwardfor', and it should insert the wanted header
[6:53pm] PiBa-NL: so basically 'yes' 

This means the PROXY protocol is only used to glue the two frontends together, encapsulating the Cient IP, but the second frontend unwraps it and saves it in the X-Forwarded-For header via option forwardfor, so that its backend can send a PROXY-protocol-less request to my app server, meaning I do not have to worry about compatibility issues up/downstream.