I could go on a rant here but I'll try to be calm. This is the classic failure of not flushing and bleeding and activating the ABS, especially on Gen1 and 2 bikes, mostly prior to '08. This should be done EVERY YEAR.
John, I'm assuming with one post you are new to the bike. Normally without ABS, the brake fluid circuit bypasses the ABS circuit and you brake normally. When you activate ABS, it forces fluid through the ABS circuit in the ABS pump. That ABS circuit has dried up fluid in it which acts like "cholesterol" in the arteries. You forced that partially hardened fluid now into the portion that goes to the rear line. Is it in the pump still or moved further, we don't know, but it is blocking the passage of fluid to the rear.
Here's a job likely to blow most of a day............
Hope and pray it moved out of the ABS pump... to help it move (big maybe) you should activate the ABS via the ABS diagnostic connector up by the front left of the battery. Harness available with a switch if you so wish, otherwise you can jumper the appropriate two wires at the ABS diagnostic connector. You will have to do this multiple times to see if you can get the blockage to move.
After that attempt, if no success, then trace the plumbing to what feeds the rear wheel. It goes through the metering block and proportioning valve, one of which may be blocked if the cholesterol made it that far. So, working upstream from the back wheel, loosen the first fitting (banjo or whatever) on the steel line and press the brake pedal to see if fluid is flowing at that point. Maybe it's just the rear line blocked? Move on to the metering valve, is there fluid there when you pump. Last point will be at the ABS pump itself, loosen that fitting and see if you have fluid flow there. Wherever you find fluid flowing, stop there and clean out the downstream plumbing with isopropyl alcohol, let it soak for a while. You'll have to hook the plumbing back up and try to get fluid moving......
If the blockage is still within the ABS pump, then I'm not sure you'll be able to fix it. Maybe with the rear line loosened/disconnected at the pump, activate the ABS again, hope the lack of backpressure will cause something to flow.
OK, all of this is going to be messy, rinse everything down with a spritz of isopropyl alcohol.
The only cure for a permanently blocked ABS pump is a replacement pump, and you have to hope you get a good used one. You don't have to replace it, just understand that ABS won't work on the back wheel.
However at the moment, fluid is not flowing to the back wheel, and that must be cured some way or another. Being that you're getting fluid flush at the right front caliper, that's a good sign, it MAY mean the ABS pump is not blocked, again, hope and pray it's just the rear line blocked. Do not ride the bike until you get the rear caliper bled.
Your rear caliper seized because (a) the blockage moved and didn't allow fluid to return, or (b) there was a lot of water content in your fluid, it may have turned to steam which pressurized the system, overpowering the rubber expansion baffle in the reservoir.