Or if you actually try to access them while being stored (eg they are in a parallel process)
That kind of thing is always a no-no in Ludii, but sometimes not obvious to see for oldies like me coming from a procedural programming background.
On the original question, if you can do it as equipment, (don't need to store information for empty sites. You can store it with (set Value at:...) but this may not be enough for what you need. - I recently packed and stored vectors in the single values in order to hold data to compare group shapes (Bug & Brood).