I too like you idea. As you already noticed, I've had similar ideas before.
Making a detailed mechanical head specification is do-able, however in order to make any attachment useful, you would need to interact with the Voltera software. For that we would need an API of some sort.
I've heard rumors that Voltera are looking at making their own attachments, so there might be a conflict of interests there, depending on their (Volteras) business model. I think a representative from Voltera can elaborate on that subject.
In case Voltera are open to the idea of 3rd-party add-ons, I'm sure you would see plenty of cool ideas realized in no time
Regarding your add-on suggestions, I think the pen is a really smart thing. I'm surprised no one has made it already. If a white ink felt-tip pen is used, you could make your own "silk screen" to facilitate component placement.
As for the via filling dispenser, I think it could be made by fitting a flexible tip to the standard syringe. There are interesting plastic material that are both (semi-)flexible and tough enough for the task. I have recently been 3D-printing with a plastic called TPU, that might be a candidate for such a tip (if I can manage to print it fine enough).
If the tip is flexible, there won't be the risk of breaking it if/when it touches the board. You could actually make it touch the surface on purpose, to create a tight coupling where conductive ink could be forced into the via hole. (think of spcecraft/spacestation docking, and you'll get the picture )
To sum it up; for any of these ideas to become reality, we need an API to send movement instructions and receive position info from. (hint, hint, Voltera )