If you click on the FieldRef link above you will see that it contains many element including the "ID" attribute.
This makes it appear that you can use these attributes like such:
<ViewFields><FieldRef ID='8bea1814-9e9e-4b3e-91c7-cd85817a2b6c'/></ViewFields>
But this does not work. It appears as if only "Name" is allowed. This is a little unusual since you are free to use the ID attribute inside the Where clause.
<Where><Eq><FieldRef ID='8bea1814-9e9e-4b3e-91c7-cd85817a2b6c'/><Value Type='Text'>Sample</Value></Eq></Where>
So, you can filter your result set by the ID of the field but not set which columns are returned. This is disappointing because I am tracking the unique Guids of my Content Type fields but not internal names.