Accessibility and multi-select in custom controls

Sorry to have to post this here, but none of the accessibility newsgroups
seem to be active.

I've got a custom control that I'm adding accessibility support to. The
control is a grid control and it supports a variety of types of multiple
selection. It can support column, row, table, or ad-hoc cell multiple
selection.

My question is, does anyone know how to handle multiple selection in terms
of the GetSelected() method of an ControlAccesibleObject derived class?
Since it only returns a single AccessibleObject, how do I handle
multi-select? Should I create a new AccessibleObject derived class that is a
container of selected objects and return that? Is that how a reader would
expect it?

DataGrid doesn't support the MultiSelectable state and neither does
CheckedListBox. The rest of the controls in .NET either don't support it or
support it in the underlying unmanaged Windows control, so I can't see any
code using Reflector.

Unfortunately, there aren't a huge number of accessibility samples and I
can't find one that supports the MultiSelectable state. I suspect not many
other people have run into this, but figured I'd take a stab at asking here
anyway. I'm often surprised by what people have done...

Thanks