A CCM_POST HTTP Request

You could define a custom ErrorDocument for the 400 Errors like so:

ErrorDocument 400 /cgi-bin/handle400.cgi

and then have that cgi (or php or servlet or whatever) do the header rewriting and location redirecting.

EDIT: because of Jacek's answer I just tested above solution to confirm it works on Apache 2.2.

My Server Config:

ScriptAlias     /cgi-bin/ "/var/www/cgi-bin/"
ErrorDocument   400 /cgi-bin/handle400.pl

lukas$ cat /var/www/cgi-bin/handle400.pl

#!/usr/bin/perl
print "Content-type: text/html\n\n";
while (my ($key, $val) = each %ENV) {
        print "$key = $val<BR>\n";
}
exit 0;

Now when accessing the broken URL that would produce an error 400 Bad Request before, I now get the following (partial) output:

...
REDIRECT_REQUEST_METHOD = GET
REDIRECT_STATUS = 400
REMOTE_ADDR = xx.xx.xx.xx
REQUEST_URI = /%&%28%7B$%7Beval%28base64_decode%28$_SERVER%5BHTTP_EXECCODE%5D%29%29%7D%7D%7C.+%29&%/
GATEWAY_INTERFACE = CGI/1.1
SCRIPT_URL = /cgi-bin/handle400.pl
...

So you do get enough information to do a proper status code change and location redirect.

GET / HTTP/1.0 is a legal HTTP request.

If there's no Host header field, you may not get the results you were hoping for if the destination server is a virtual host that doesn't have its own IP address to distinguish itself from other virtual hosts.

HTTP 1.1 requires the Host field.