Is it possible to access the clientId from an authToken (RequestContext) in WebAPI 2

When logging in you can set a claim on the identity in the GrantResourceOwnerCredentials

identity.AddClaim(new Claim("oauth:client", client));

That way it is available once the User Principal's Identity has be set.

You can create an extension method to extract it conveniently

public static class GenericIdentityExtensions {
    const string ClientIdentifier = "oauth:client";

    /// <summary>
    /// Set the client id claim
    /// </summary>
    /// <param name="identity"></param>
    /// <returns></returns>
    public static bool SetClientId(this IIdentity identity, string clientId) {
        if (identity != null) {
            var claimsIdentity = identity as ClaimsIdentity;
            if (claimsIdentity != null) {
                claimsIdentity.AddClaim(new Claim(ClientIdentifier, clientId));
                return true;
            }
        }
        return false;
    }        

    /// <summary>
    /// Return the client id claim
    /// </summary>
    /// <param name="identity"></param>
    /// <returns></returns>
    public static string GetClientId(this IIdentity identity) {
        if (identity != null) {
            var claimsIdentity = identity as ClaimsIdentity;
            if (claimsIdentity != null) {
                return claimsIdentity.FindFirstOrEmpty(ClientIdentifier);
            }
        }
        return string.Empty;
    }
    /// <summary>
    /// Retrieves the first claim that is matched by the specified type if it exists, String.Empty otherwise.
    /// </summary>
    public static string FindFirstOrEmpty(this ClaimsIdentity identity, string claimType) {
        var claim = identity.FindFirst(claimType);
        return claim == null ? string.Empty : claim.Value;
    }
}

So now once you have the the user principal you can extract the client id from the claims.

var currentUser = RequestContext.Principal;
var client = currentUser.Identity.GetClientId();