普通にxhrを弄る場合は

http://www.w3.org/TR/XMLHttpRequest/

http://www.w3.org/TR/XMLHttpRequest/#the-setrequestheader-method

mimetypeなどは

はAかBで更新できたのですが、この辺はダメで

さてどうしようかと webRequest API の登場です。

http://code.google.com/chrome/extensions/trunk/webRequest.html#event-onBeforeSendHeaders

* onBeforeSendHeaders (optionally synchronous)
Fires when a request is about to occur and the initial headers have been prepared. The event is intended to allow extensions to add, modify, and delete request headers (*). The onBeforeSendHeaders event is passed to all subscribers, so different subscribers may attempt to modify the request; see the Implementation details section for how this is handled. This event can be used to cancel the request.
* onSendHeaders
Fires after all extensions have had a chance to modify the request headers, and presents the final (*) version. The event is triggered before the headers are sent to the network. This event is informational and handled asynchronously. It does not allow modifying or cancelling the request.

この２つは、header書き換えできるらしい？

chrome.webRequest.onBeforeSendHeaders.addListener(function(object details) {…}, RequestFilter filter, array of string extraInfoSpec);

manifest.json permissionsにwebRequestを追加

  "permissions": [
    "tabs",
    "webRequest",
    "http://*/*",

chrome.webRequest.onBeforeSendHeaders.addListener(
    function(details) {
      // console.log( details.requestHeaders );
      for (var i = 0; i < details.requestHeaders.length; ++i) {
        if (details.requestHeaders[i].name === 'User-Agent') {
        　details.requestHeaders[i].value = 'MY REQUEST';
       }
    }
    return {requestHeaders: details.requestHeaders};
    },
    {urls: ["http://*.bbtune.com/*"],types:["xmlhttprequest"]},
    ["requestHeaders"]
    );

eventは発生していますが書き変わらず？

onBeforeSendHeadersで書き換えた後に onSendHeadersで取得できるdetailsは

のようにちゃんとmy requestに変わってるのに・・・サーバーに送られるヘッダーが変わってない・・・

xmlhttprequestのときは、変更出来ないっぽいです。
裏にtabを開いてみたりして、ajaxじゃない方法でデータを取得してみたりします。