There is no supported way of changing OWA settings using WebDAV for Exchange 2007. Under Exchange 2000 and 2003, there were properties on the mailbox root which could be changed via code. However OWA 2007 is a much different animal.
Under Exchange 2000 and 2003, there are properties which you could get at with WebDAV using the http://schemas.microsoft.com/exchange/ namespace. While you may be still able to modify these using WebDAV under Exchange 2007, you will find that they won't affect OWA. Some examples of these are:
There are 'Set' and 'Get' Exchange Web Services (EWS) which do most of what developers want under Exchange 2007. These web services are for User availability and OOF setttings. Note though that there is no API which is for modifying OWA properties - so there will be some limitations on what can be done with Web Services.
PowerShell (Exchange 2007) or CDOEXM are more mailbox/store oriented and are not geared to modifying item/folder/OWA properties. CDOEX in most cases can get to the same properties as WebDAV.
Now, if you want to do some general OWA Customization (ex: tweak the UI) or add-in your own custom forms for your own message types, this is possible for OWA 2007 SP1. This ability is not in the RTM version of OWA 2007.
Whats new in Exchange server 2007 SP1
Customizing Outlook Web Access
Introduction to OWA customization
OWA forms registry
Outlook Web Access User Interface Customization XML Elements