I am not sure I would go this route but you already started with the menu bar.
So using the reference of:

http://www.nobsphotosuccess.com/openletter

I would start with the template editor in the ubb control panel.

Login.tpl and logout.tpl and not_right_bare.tpl and notright.tpl

find the line that has
<a href="{ubb url="ubb=newuser"}">{$lang.REG_ONE}</a>.

and replace it with

<a href="http://www.nobsphotosuccess.com/openletter/">{$lang.REG_ONE}</a>.

I think that is all you have left to change.
I am not sure how you accept registrations except for you manually add them to the ubb database. Again why not use the subscription feature built into UBB..

You still need to modify the language files for changing the message verbiage.
Also you might consider completely closing the board from guests with the global values.

Blue Man Group
There is no such thing as stupid questions. Just stupid answers