Remove "[External]" tags added by email server - gmail-api

Question

My organization has begun adding obnoxious warnings to the subject and body of any email that arrives from an external address. My organization allows email forwarding, and I forward to gmail. I would like to have the extra text automatically removed before I see them in gmail. This would not violate the policies at my organization. Some ideas I had:
(1) One option would be to set up a gmail filter that edits the message. This feature does not seem to exist.
(2) Another option would be to customize the display so that this particular text is never shown on the screen. Again, there seems to be no way to do this.
(3) I wrote a script in Google Apps to grab to the content of each offending email and send an edited version to myself, but there is no way to make the "from" field show the original sender (perhaps with good reason). I can put that information in "reply to" but the gmail client doesn't show it nicely. This removes the annoying extra text at the cost of the ability to easily see who sent the email. The core of it is this call:
GmailApp.sendEmail("<my_email_address>",newSubject,newBody,{
attachments: message.getAttachments(),
bcc: message.getBcc(),
cc: message.getCc(),
htmlBody: newBody,
replyTo: message.getFrom(),
});
Any ideas? This is becoming a big problem as the "nanny state" approaches, so I'm sure others will appreciate your solution.

Answer 1

Issue
It unfortunately seems that at the day of this post, there is no way of edditing a forwarded email with the Gmail API. Check the documentation to see what you can actually do with the Gmail API in regards to forwarding email addresses.
Workaround
Despite it is not possible to edit a forwarding email address, in your case you are interested in:
Edit the body and subject of the email address.
Preserve the information of who the sender was.
Therefore what you could do is:
Get the email address you want to edit. Get the body, subject and information about who was sending it and store that.
Create a new message where you edit the message you just got and write on the subject the from field that was in the message you just got.
Send that created message to your desired recipient just like it was a forwarding email.
I hope this has helped you. Let me know if you need anything else or if you did not understood something. :)

Answer 2

Apps Script
You can insert messages into your inbox with their original "From" field but a modified body.
Note: Inserting is different from sending because the message is created on the inbox with less validation than sending.
To use this on Apps Script, take a look at the Advanced Gmail Service.
Custom Application
Another idea on how to do this is to use a custom application (in any language you'd prefer) that:
scans your company inbox with IMAP
manipulates the message body to remove the artifacts you don't want
insert manipulated message into your Gmail inbox with the Gmail API