You can remove the rack without removing the rad… but you CAN, if you want to, replace the boots and refill the rack with it still in the car. Jack the car up on one side, and then pull the small diameter of the low side rack boot away from the steering rod, so that the fluid drains out. The oil will run along the steering rod, so you may need a large catch pan. While you’re letting the fluid run out that side, undo the high side steering arm and replace the rack boot. Snug up the boot clamps on that side.
After the fluid finishes draining from the low side, put the car back on level ground and then jack up the other side, and replace that boot. If you first snug up the large diameter (inside) clamp on the rack boot, you can use a tapered nozzle on your gear oil jug to pull the outside end of the rack boot away from the steering rod, and get the oil in that way.
I had to fix a hole in the center of the rack tube on my Tiger. The PO had used a too-deep crank pulley, and it rubbed through the rack tube, causing the oil to leak out. A second issue was that they’d run the remote filter oil hoses past the left hand rack boot, which caused it to wear out prematurely.
So I ended up doing the boot replacement with the rack out of the car, but I did fill it by squeezing the oil filler past the small end of the rack boot. IIRC I just used the "calibrated" level lines on the fill bottle to judge the amount of oil I put in.