Existing website written in PHP Zend Framework
We've successfully implemented a setup that will solve your issue with several sites. Unfortunately we haven't been able to open source things yet, but this is our general process:

We set up a Craft install on a separate domain (or subdomain, in our case). Then we serve the content as JSON instead of html in our layout template. 

Our entry templates are totally normal, outputting html like any other template does. This layout just wraps the whole thing in a json object (and does not include the header/footer etc).

Then on the ZF side, we wrote a module to pull in the data from the separate domain and output it in our app, using the normal layouts that already existed. It is also obviously heavily caching the content it receives from Craft.

So, ZF is actually handling all of the requests by end users, and the only things going to Craft are actually curl requests from ZF.

We have ZF send along a Header telling Craft that the request is coming from ZF, and then we have Craft check for that header. If it exists, it uses layout.json, and if not, it uses layout.html.

layout.html has a dummy version of the header and footer, and is only used for Live Preview basically. The layout.json file serves our content as json to the ZF app.

Here is our layout.json:

{% header 'Content-type: application/json'
%}{% set response = {
    success:true,
    metadata: {
        {# some custom data #}
    },
    data: {
        meta: (head is defined ? head : {}),
        body: block('content')
    }
} %}{{ response|json_encode()|raw }}


So, while this doesn't mess with custom routing, it will allow you to keep your ZF app going and have Craft handle marketing/static pages.