I've been thinking of adding the root directory to the load path as well. I do this https://github.com/rejeep/wrap-region/blob/master/features/support/env.el#L6 and this https://github.com/rejeep/commander.el/blob/master/test/commander-init.el#L5-L11 in most my projects. If the root directory was added to the load path in `cask exec` it would solve this.

It's however important to add the directory at the end. When testing `f.el`, I use `ert-runner`, which depends on `f`. But I don't want ert-runner to use the local `f`, but the one from Elpa.