I never use the built-in grid toolbar, I always create my own full Kendo ToolBar and put it into the grid's toolbar area as a template.
This way, I am in full control of all aspects of the ToolBar and can put whatever I want in it, hide/show/disable/enable buttons as I see fit.
You may also be able to get a reference to the buttons you want to affect via something like
$("#myGrid > .k-grid-toolbar > .k-button.k-grid-add")
and then add/remove display:none or the disabled attribute in response to your readable/modifyable button click.