Yes they can, and there are a few approaches to this.
The approach I use is to create public methods within the master page that will do the modification/access to the data within the master page. For example, I typically like to modify the link style of the current page/category I am on, so I have a method in my master page like this:
Public Sub SetNavigationPage(ByVal MenuName As String)
DirectCast(Me.FindControl(MenuName), HyperLink).CssClass = "MenuCurrent"
End Sub
Then in my content page, I simply access this method as such:
Dim myMaster As EAF = DirectCast(Me.Master, EAF)
myMaster.SetNavigationPage("hypViewEmployee")
...where EAF is the name of the class of my master page.
One interesting issue I've found is that I've had complications with using the Visibility property of .NET controls when trying to show/hide them in this manner. This is due to the rendering orer of master and content pages. To resolve this, I setup a basic CSS style for both visible and hidden and set the CssClass property accordingly.