How to tent a via without changing DRC file in EAGLE
The simple solution is as follows.


Set you Mask Limit and run DRC to apply it.
Group everything on the board, then select Stop-&gt;On from the Change tool (button looks like a Spanner/Wrench).
Apply that to the whole group (right-click in an empty space on the board somewhere and select Group:Change). This will ensure that every via on the board has a stop mask aperture applied.*
Now select Stop-&gt;Off from the Change tool
Click on each via individually that you want to be tented. This will remove the aperture in the stop mask for that via individually.




*Note: Applying to the whole group with Stop-&gt;On shouldn't cause you an issue. The attribute won't affect components (and crucially pads within them), so any stop mask settings within a component will remain as they were. The only thing as far as I know that responds to the Stop attribute is vias, and only those which are not excluded by the Mask Limit in the DRC.