Has anyone been able to implement a custom login page to their Nimsoft On Demand UMP?
Yes, modifying this is easy, you just need to hover over the UMP main menu drop-down (in the upper right hand corner), go to My Places, then go to the Public Pages, within here select your logo in the upper left hand corner and it will expose the main login page, now you can freely edit it the same as you would any other page.
Once I have modified that page, how do I ensure that every new user I create gets that page instead of the default?
That is the default public page that your redirected to so unless you have direct links to other communities everyone will go here.
Retrieving data ...