Hey! Sorry but I'm going to have to correct you here, what you're describing didn't start until the original Xbox.
First of all, EPROM PCBs work fine in retail systems, and there's just no such thing as a special review system for cartridge-based games (other than the capture units for portable systems that you described). CD-based systems (post-Sega CD) had copy protection, and so there were special debug units to play burned discs, but nothing about them did any video capture. As you mentioned, portable systems had special units for capture, but until the DS these special units just spit out standard television video.
How editors captured screens depends on the publication, year, and environment. Nintendo Power took photos of a CRT in a dark room until about issue 20, when they switched to a video printer, before finally settling into using a video capture card on a computer (I'm not sure when this switch occurred, since video printer stuff looks so similar to digital capture). They almost always captured in pure RGB, and had modified NES units to do this. EGM also took photos at first, and is famous for its photos at trade shows, but for in-office capture they switched to capture cards on Macintosh somewhere around issue 9 (and like Nintendo, they modified their systems for RGB video to get the best quality). They also would sometimes record gameplay onto video tape and capture the frames they wanted afterward.
If you want to go back further than Nintendo Power, most of the early 80s stuff was using publisher-provided screenshots, most of which were actually hand-drawn and not actually capture of a game. Computer magazines took photos, either directly or with a really neat device called a "film recorder," which was basically a tiny CRT inside of a box with a 35mm camera attached. I don't think video game magazines used these, but I think some PR/marketing/back-of-the-box shots did.
The switch to digital capture came at around the same time that magazines switched from paste-up layout to digital. Pretty much every video game magazine was using video capture cards and doing all-digital layouts in QuarkExpress by 1993 or 1994, give or take, and did so until the systems did their own internal capture. At least some (and maybe most/all) of them usually had a foot pedal so they could easily capture screens while playing, instead of having to reach over and hit something on the keyboard.
The first system to have any kind of frame buffer capture, as I mentioned, was the original Xbox debug kits. You had to network it in to a computer and hit a capture key there. It became standard in the 360/ps3 era, just as it became standard for users.