I went back to look at pictures of J-20 in various stages of production. I am not sure but I have a proposal of what is done.Maybe a nit pick, but why are there so many rivets on the canopy?
I know they will apply RAM coatings, but could they maybe not have designed it so the rivets are on the inside of the canopy to keep the external skin as smooth as possible?
With the canopy, it’s not like they would need to regularly undo the screws for regular maintenance and check ups to start with.
Even if you do need to undo the screws, that will mess up your stealth coating, so I am really struggling to understand why they didn’t just design the canopy frame such that all the rivets are installed from the inside.
I think these holes are for fine finishing by adhering some kind of plastic/rubbery strips. The strips are heated to very high temperature becoming jelly like when being applied. The strip is then applied and pressed to the area with those small holes. Because it is in a jelly state, the strip material will be pressed into the holes and fill the interior which has a bigger cavity. After cooling, the strip will stay tight on the surface. The small holes are essentially for self-riveting of the strips. It is actually what you have suggested "inward rivet". Only the bigger holes are meant for screws for maintenance be left open. This can be seen that in the 2nd and 3rd photos that there is no marking of the smaller holes but only the recessed marking of the bigger ones. The reason of doing this is probably for RCS reduction, covering all gaps smoothly.
Here are the photos with my markings. They are large photos, so you can zoom in and take a closer look.
Very rough and big gap around the light strip. The canopy base are joint by four pieces with gaps. This is impossible to be finished with thick paint.
In the semi-finished state, no sign of small holes. Only big ones. The marked area are otherwise super smooth.
In the finished state.