Zenpress I believe requires the ability to access zp-config information, and as such, would probably not work, and if it did would be efficient to do so across multiple ISP linkages.
If you are planning on having lots of traffic, a local install would be best anyway, as -theoretically- what happens if one ISP is down, and people cannot get to the other system. For example, your wordpress blog running on rackspace, has a zenshow or zenpress plugin installed, but your goDaddy account is down. Then, neither you nor your internet customers will be able to access pictures.
I'm coming from a network administration standpoint and it's a good practice to always want to make sure you have your resources in the same spot for both security and performance reasons.
This is just a suggestion, I would "pilot" or test the solution first using a demo database setup, and see what your results are.