Figured it out, posting answer here in case anyone has the same issues.
The key is that you have to specify the namespace of the element you're parsing, even if the XAML does not have the namespace for that element explicitly declared. In my case, adding:
mgr.AddNamespace("x", "http://schemas.microsoft.com/winfx/2006/xaml/presentation");
And then referencing the RowDefinition elements as:
xdoc.SelectNodes("//x:RowDefinition", mgr)
Was able to pull it off.