Extend sanely (Score 2, Informative)

First off, I totally agree with Samschnooks that as a development team, that 5% is your responsibility, and what you're really getting paid for. But given your concerns about maintainability, and what I'm reading as your concern that custom code that you create may end up being addressed by the framework, I do have some advice.

See what of your own code can be crafted into a plugin to extend the framework. Rails plugins are quite easy to create and insanely easy to use.

Be diligent about abstracting the functionality that you need, keeping domain-specific business logic out of the plugin and in the application instead. You'll find that plugins are easy to write test cases for and will keep your custom logic very modular.

If someone else ends up releasing a plugin that addresses your needs, you can simply swap it out; the same applies if the framework gets extended to include what you need.

If maintenance is a real issue for you, release the plugin yourself as open source and recruit some assistance from the community. Chances are, you're not the only one in the world who needs the functionality.