My personal perspective:
First off, in my opinion, a hard-and-fast rule is not appropriate here. Voting is private, so a rule banning voting not enforceable. There's no point in making a rule that can't be enforced. Also, what makes sense will depend on the context, so a bright-line rule is not appropriate. Instead, I think it makes the most sense to focus on norms and principles, and what we'd advocate for.
Our general principle is that of self-governance: a site is run by the site's community, and we want policy decisions to be made by those who are part of the community, rather than by outsiders. We'd also like people to be informed about the issues before voting. Generally speaking, we'd probably discourage people who aren't active in the community and aren't informed about the issues from voting on policy questions.
Another principle is that of self-governance: empirically, communities seem to work better when they make their own decisions and are given authority and empowered to do that. So, if SE employees vote on a heated issue, there's a risk that this sense gets violated, and that can lead to dysfunctional results. I would hope that SE employees will take this into account before taking a position.
But I think it's also important to recognize that there are situations where SE employees may have an important and valuable contribution to make. They may bring useful experience from other sites, or a helpful perspective. It would be great to have them involved in those cases.
I also think it doesn't make sense to lump all SE employees together. To give an example, there may be a significant difference between a CM vs a marketing representative. A CM is likely going to bring a lot of experience in related matters, and I'd love to have CMs participating in our meta discussions.
As a general rule of thumb, I think we should trust SE employees to self-assess the situation, and welcome and encourage them to participate in meta discussions and upvote or downvote posts if they feel that the benefits outweigh the risks. If it becomes a problem, we could discuss, but I don't think it's worth setting a more specific general rule.
Lest it be unclear, I think it's absolutely appropriate for SE employees to participate in meta discussion by posting questions, answers, and comments.
Finally, I'd like to provide some context. You mention "using voting powers, granted only as part of the employee mod diamond and not earned as privilege" -- I think some perspective is in order here. It only takes 101 reputation to have the privilege to upvote, and 125 reputation to have the privilege to downvote. Are we really arguing about a privilege that's granted to people at 101 reputation, or 125 reputation? There's nothing magical about the reputation threshold; it's a heuristic for identifying people who have some experience with the site and some exposure to the model. Indeed, you can gain 101 reputation via the association bonus with no experience or activity on the site at all, as long as you have some activity on some other Stack Exchange site. I think it's likely that most SE employees are going to have that at least as much as someone who uses their association bonus to upvote a post on Meta, or a new user who just received 125 reputation, so I can't see a big deal here with SE employees having that privilege.
Related: Should Community Managers remain impartial during Moderator elections?.