Patent agent work from the research career perspective
I am wondering about whether helping those people to formulate their ideas in a clear and defensible manner can be counted as an informal co-authorship of some kind.


My impression (based on my own experience as well as what I've heard from others) is that patent attorneys or agents typically make no contribution at all to helping their clients formulate ideas in a clear and intellectually defensible manner.  Instead, the contribution is entirely on the legal side: formulating things in a way that is likely to pass the review by the patent examiner and any subsequent legal challenges, while at the same time claiming as much intellectual property as possible.  This is a very valuable contribution, but not the sort of value that most of academia rewards (it's simply orthogonal to the intellectual contributions).  It's possible that a law school or business school might specifically hire someone because of this experience, but for most departments experience as a patent agent would not count as anything remotely like research.  

You could try to make a special argument for your case (maybe your activities would differ from those of the average patent agent), but you would face a lot of skepticism.


  In other words, is there some ethical authorship credit that would be attached — again, informally — to patent agents' research portfolio and enable them to further return back to mainstream academia at some point?


This is no such convention, and you would have to be very careful if you made any suggestions in this direction (since it could easily come across as trying to take unfair credit for other people's work).

I don't have any statistics, but I believe returning to academia after spending time as a patent agent is rare.  I've known several people with Ph.D.s who became patent agents, and I've known of others, but I've never heard of anyone returning to academia afterwards.  (I wouldn't be surprised if it has happened, though.)


  Considering the above-mentioned point and, perhaps, other aspects, how difficult would be for a Ph.D. graduate without significant research portfolio (at the moment) to return to academia (as postdoc, then faculty) after having worked as a patent agent for some time?


It depends on what sort of academia.  Let's assume you wouldn't be teaching law or business, so those aspects of your experience would not be relevant.  You could still make a case that your time as a patent agent gave you an unusually broad perspective on how your field is applied, and this could be attractive to departments.  However, under ordinary circumstances it could not make up for a lack of recent research publications.  If you keep publishing or apply to jobs without a strong research component, then you may be OK, but if you stop publishing, then it will be difficult to return to a research university, even as a postdoc.