The inplace editor is a TTMSFNCEdit (TTMSFNCDataGridEdit), with special handling for numeric fields, etc… You can use the following code to detect and cast:
procedure TForm13.TMSFNCDataGrid1GetInplaceEditorProperties(Sender: TObject;
ACell: TTMSFNCDataGridCellCoord; AInplaceEditor: TTMSFNCDataGridInplaceEditor;
AInplaceEditorType: TTMSFNCDataGridInplaceEditorType);
begin
if AInplaceEditor.IsEdit then
begin
AInplaceEditor.AsEdit;
end;
end;