Is there any way to configure IIS 7.5 to perform URL rewrites to different application pools on the same site without running into a 403.18 error?

We're using Helicon ISAPI Rewrite 3 on IIS 6 and it's working like a charm. The root-level "application" is running under it's own application pool, and on IIS 6 we have no problems doing URL rewrites from that application pool to any one of the other four application pools. But when I copy the same server configuration information over to IIS 7.5 the URL rewrites to any of the other application pools fail with a "403.18 -- Forbidden" error.

The weird bit is that the IIS 6 is not (at least as far as I can tell, by looking at the site Service configuration dialog) running under IIS 5 emulation mode, so somehow the rewrites aren't throwing 403.18 errors. So something must be different... but whatever it is, I sure haven't been able to figure it out.

Btw, we're not married to Helicon ISAPI Rewrite. If there's another way to preserve our current rewrite configuration rules using another module or method I'd be more than happy to use it.