Currently the only part of the My Account page that is easily customizable is the content of the Profile tab as documented here.
I don't recommend you modify the mojoportal code because then it will be difficult to get upgrades. The best strategy I can think of if you need to be able to customize it, is implement a custom version in a separate web app project (add your own project(s) to the solution) and use a post build event to copy your version up to the main Web project replacing the existing page.
Hope it helps,
Joe