You can use the default checkbox template and provide the custom check functionality when checkboxes are checked. However, the loadOnDemand + checkboxes scenario requires you to handle checked items that do not have loaded subnodes in some way - the subnodes will not be checked in the treeview, because they are not loaded.
The following jsBin should help you with implementing this functionality. Please note that it also shows a bug that has been fixed in the latest internal builds (it always updates the indeterminate state, even though chechChildren has not been set).
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