Is it appropriate to bring to an editor's attention non-ethical publishing that can't be directly proven? The situation concerns ephemeral facts, that can't be proven to an department outsider. 
The sole two authors of an accepted paper have virtually no contribution to the result, but both hold prominent positions of authority within the department and are on top of that have close family ties with each other. 

The main theoretical research was done by two computer scientists and the experimental setup &amp; execution by a whole team of software engineers. Both groups are routinely engaged in R&amp;D, but are affiliated with a company, that is owned by the two authors.

These affiliations are nowhere stated in the accepted version of the paper. And there is no chance that anyone will publicly come forward for fear of retribution. So, there is literally no physical evidence. 

However, I feel that the conditions warrant that the editor of the journal that accepted the paper be made aware of the circumstances.

I stand to gain nothing from either accepting or retracting the paper, nor am I employed by said company, but I am affiliated with the department and feel that such behavior corrupts out department and science as a whole.

So the question is whether it is appropriate to write an email to the editor stating the above and additional information?
Contacting the editors may not have an effect at all. They simply cannot tell whether your accusations are facts or are just made up.

If you really want to do something about this, you should rather write to their institute or university and inform them about the suspected scientific misconduct. They often have specialists nowadays who will try to resolve such issues. Be prepared to be asked for proof.

Please note, that it might be completely valid to only have these two authors: authorship is not automatically gained by implementation work but rather by significant contribution to the paper. Please check which criteria must be fulfilled in your institute or field to become an author of a paper.