We use the XamDataGrid a lot for input, and we wanted to do something similar to the XamMultiColumnCombo editor. Basically in the text box you can enter labels, values or a formula -- the helper/editor beneath the text box shows information relative to what they've typed in thus far.

We implemented it with an adorner using the custom behavior, but the downside to adorners is that they cannot show outside certain boundaries like the bottom of the WPF window, and it's more complicated to know when focus is lost from the cell (to know when to close it).

I would suggest creating a custom editor functionality that would make such things easier.  Something like being able to have a combo editor (or a XamMultiColumnComboEditor), but with a blank slate I can put my own content/control into.