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:

    http://schemas.microsoft.com/exchange/weekstartday
    http://schemas.microsoft.com/exchange/workstarttime    
    http://schemas.microsoft.com/exchange/workendtime
    http://schemas.microsoft.com/exchange/signaturetext
    http://schemas.microsoft.com/exchange/signaturehtml

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. 

    GetUserAvailability Operation
    http://msdn2.microsoft.com/en-us/library/aa564001(EXCHG.80).aspx

    WorkingPeriod
    http://msdn2.microsoft.com/en-us/library/aa580377(EXCHG.80).aspx

    WorkingHours
    http://msdn2.microsoft.com/en-us/library/aa565001(EXCHG.80).aspx

    GetUserOofSettings Operation
    http://msdn2.microsoft.com/en-us/library/aa563465(EXCHG.80).aspx

    SetUserOofSettings Operation
    http://msdn2.microsoft.com/en-us/library/aa580294(EXCHG.80).aspx

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
    http://technet.microsoft.com/en-us/library/bb676323.aspx

    Customizing Outlook Web Access
    http://msdn2.microsoft.com/en-us/library/bb891801.aspx  

    Introduction to OWA customization
    http://msdn2.microsoft.com/en-us/library/bb891803.aspx  

    OWA forms registry
    http://msdn2.microsoft.com/en-us/library/bb891879.aspx  

    Outlook Web Access User Interface Customization XML Elements
    http://msdn2.microsoft.com/en-us/library/bb891846(EXCHG.80).aspx