Exchange IP-STS JWT token for ACS JWT token

I have an azure ACS service, which trusts an IP-STS.
For an active scenario, I first get a JWT token from my IP-STS using username, password credentials. There is Oauth2 endpoint and everything works quite well. Is it possible to "exchange" this IP-STS token for a JWT token issued by my azure ACS ? If so, is there an example of code that does this. (To make things worse, all my code is in JavaScript (actually TypeScript) but that doesn't really matter).

Update : I'm working on your hint with the ACS OAuth2 draft 13 endpoint.
I proceed as follows : I ask my custom STS (ThinkTecture STS) to give me a JWT token for the "ACS OAuth2 draft 13 endpoint" realm. This requires an oAuth client id and secret which are global in the TT STS and I assume they are irrelevant. In the TT STS management I have a symmetric key configured for this realm : key1. I receive the 3-part JWT token. The signature on the token is indeed made with key1. I then pass this token to ACS with client id and secret from a service identity and parameters as specified

var form = new FormUrlEncodedContent(new Dictionary<string, string>
{
  { "grant_type", "http://oauth.net/grant_type/jwt/1.0/bearer" },
  { "assertion", rawtoken (the header dot body dot signature form TT STS },
  { "scope", "http://localhost"}
});

Unfortunately I now get {"error":"invalid_client","error_description":"ACS50027: JWT token is invalid. \r\nTrace ID: b107cda5-393b-4b50-b14a-ebaa0ac41913\r\nTimestamp: 2012-12-05 08:58:10Z"}

I understand JWT is in beta and therefore ACS50027 is not yet documented. The hard part is that there is no known way to debug this. Thanks for nay help.

Solution

Ok, I finally found a way to do this. Your hint was crucial for the success. The (very) tricky part is that the IP-STS uses a symmetric key to sign the JWT token. I had to programmatically (using the odata api) insert this key in the ACS backoffice under the Identity provider. (the managament console does not show this key anywhere) Everything is nearly fine. The only trouble is that the ACS copies the audience ("aud") from the original token, while I request a token for a different audience ("scope"). Any ideas there ?