Multiple Services on One SP

[Jason]

Hi All:

I'm working to set up integration between a Shibboleth IdP and a
service provider that hosts multiple web applications.

That is, the service provider has a single ACS URL, and the webapp
that the end user will be redirected to depends on the value of custom
attributes that I pass in the SAML assertion.

Since we're using an IdP-initiated login flow, end users would
normally be given a URL like this to get to a web application:

https://idp.domain.com/idp/profile/SAML2/Unsolicited/SSO?providerId=[SP ACS URL]

The problem is that I'm not sure of a good way to control the value of
the custom attributes except by using the attribute resolver.

For example, if the "Application" attribute has a value of "2" in the
SAML assertion, the end user would be redirected to application #2 on
the service provider.  Is there any way that I can specify what the
custom attribute value should be using some value in the URL?

I know something like:

https://idp.domain.com/idp/profile/SAML2/Unsolicited/SSO?providerId=[SP
ACS URL]?Application=2

wouldn't work, but is there any other way to accomplish this?

--Jason