iRule ReWrite - Inserting JSON Parameter into Request + Removing Same JSON Parameter from Response

Yes, iRules LX would be a good option as it can handle JSON objects natively.

Here's a sample code. It adds the key-value {"newKey":"new"} to the request data (POST), and removed the key "aoc" from the response body. This is just a skeleton code. You can add any conditions you want (such as checking the presence of a certain key etc).

iRule part

when HTTP_REQUEST {
  set RPC_HANDLE [ILX::init RemoveKeyPlugin RemoveKeyExt]

  if { [HTTP::method] eq "POST"} {
    HTTP::collect [HTTP::header "Content-Length"]
  }
}

when HTTP_REQUEST_DATA {
  set payload [HTTP::payload]
  set req [ILX::call $RPC_HANDLE req $payload]
  log local0. "$payload > $req"
  HTTP::payload replace 0 [HTTP::header "Content-Length"] $req
}

when HTTP_RESPONSE {
    HTTP::collect [HTTP::header "Content-Length"]
}

when HTTP_RESPONSE_DATA {
  set payload [HTTP::payload]
  set res [ILX::call $RPC_HANDLE res $payload]
  log local0. "$payload > $res"
  HTTP::payload replace 0 [HTTP::header "Content-Length"] $res
}

Node (extension) part

var f5 = require('f5-nodejs');
var ilx = new f5.ILXServer();

var parser = function(str) {
  var json = {"error":"unable to parse"};
  try {
    json = JSON.parse(str);
  }
  catch(e) {
    console.log('Got non-Json data: ' + e + ' >>> ' + str);
  }
  return json;
}

// Adding an entry
var addEntry = function(req, res) {
  var json = parser(req.params()[0]);
  json['newKey'] = 'new';      // Adding a new entry
  res.reply(JSON.stringify(json));
};

// Removing an entry
var removeEntry = function(req, res) {
  var json = parser(req.params()[0]);
  delete json['aoc'];
  res.reply(JSON.stringify(json));
}

ilx.addMethod('req', addEntry);
ilx.addMethod('res', removeEntry);
ilx.listen();

Example output

Jul 17 16:59:04 ltm1310 info tmm[17714]: Rule /Common/RemoveKeyPlugin/RemoveKeyRule : {"hello":"world"} > {"hello":"world","newKey":"new"}
Jul 17 16:59:04 ltm1310 info tmm[17714]: Rule /Common/RemoveKeyPlugin/RemoveKeyRule : {   "name":"Chateau Mouton Rothchild",   "aoc":"Pauillac",   "year":1978 }  > {"name":"Chateau Mouton Rothchild","year":1978}