The following worked for me, but I’d welcome any input from Team GFC.
I had to remove the steel tube to address the pins when they rusted in place. If the pins still slide freely, it’s not too difficult with a few tools. If they’re stuck fast, they need to be loosened up enough to be driven inside so they clear the ends of the tube, but not so far that the 1/4-20 threaded hole is covered.
I removed the latch screws and replaced them with a 1/4"-20 bolt that I could tap on gently. I liberally applied Ballistol lubricant to the pin; this worked better than Boeshield T-9. I drove the pins in with a 2 lb hammer and 3/4" diameter drift punch, stopping to hose down the pin with Ballistol before each stroke. I found that the shock of metal-on-metal contact worked better than using a dead blow hammer during the initial cycles.
After several minutes of driving the pin in with the punch, lubricating, and tapping against to bolt to drive it out, I was able to free up the pin enough to move it by hand. I turned the pins on a lathe to slightly reduce their diameter, which allowed more space inside the tube for grease and a looser fit. Adding a hole to the middle/bottom of the tube allows much easier access for introducing lubricant with a long straw.
Once the pins were driven inboard enough, I removed all of the screws holding the milled end brackets and the T-slot frame together. Use a P3 size screwdriver. Not wanting to chance damaging the heads, I used a Vessel Megadora P3 JAWSFIT screwdriver. The tip fits precisely, and the serrations bite into the head rather than camming out. It may be possible to leave some of the screws installed while spreading the frame, but I decided to have the connections fully loose rather than risk damage while tapping it apart. Any good P3 screwdriver will probably get the job done.
I used a 1 lb dead blow hammer to separate the milled ends apart from the tube. It doesn’t take much space to gain the necessary clearance, so there’s no need to fully separate the frame parts.
Once the tube is removed, slide the latch pins out. If one is still stuck, a piece of pipe can be inserted from the opposite end and the tube can be driven down on the pipe, driving out the pin.
I thoroughly wire brushed the inside of the steel tube, and applied a heavy coat of Fluid Film rust inhibitor to prevent another rust-up.
Re-assembly is straight forward; basically reverse the above steps. Rather than trying to beat the frame & ends back together, or draw the parts together with the screws, I used a furniture clamp to bring the parts together. You can check alignment by closing the upper & lower frames together, and by measuring corner-to-corner diagonally across the top, but it should be square once the parts are fully seated.
It’s a little fiddly trying to keep the latch tube in position during assembly, so a helper will make the job easier. I’ll give the new aluminum parts a try when they become available.