I don't think that a higher dpi printer would help. Probably it would just aggravate the problem.
My guess is that an injet might be able to lay down a smaller dot than a laser, but I'm not sure
about the "repeat accuracy" of the printed pixel size from dot to allegedly same size dot.
Perhaps there is an HP inkjet expert that can comment on this.
My experience will LaserJets is that if you take a typical grayscale 300 dpi image that prints fine,
and then if you print that same file but instruct the printer to print at 600 dpi, the new image is of course
smaller but DARKER.
In terms of "photograpic printers" I was thinking of the high end scanners used in the printing industry
to create the printing plates. These things were originally in the $500K - $1000K price range. They print
with light onto light sensitive media. If you know somebody in the printing, they might be able to point you.
The other thing you might look at is a thermal printer that uses heat to blacken the paper; e.g.,
many barcode printers. I doubt that a thermal printer that uses a ribbon would we even be as good as
your laser. The thermal printers that directly heat the special paper may not have sufficient resolution
for your purposes and may suffer from the same problem.
There were color printers that made 3 or 4 passes over the paper with CMY and maybe K I can't remember, but
I never saw output that needed a 600dpi output.
Hopefully, someone with experience in this area will come forward.
Chris H., any thoughts?