OK, your first statement tells me the basics of your tree's problem - the pot has no drainage. Putting rock in the bottom is, unfortunately, a myth. What happens is that the rock raises the bottom level of the soil, thus raising the 'perched water table' - there's a certain area of soil at the bottom of the soil mass that holds water after the main quantity of water has drained. It never drains down, it can only move through the soil in an upward direction, either to the roots or to evaporate into the air. It sounds weird, I know, but it's what happens. The depth of the perched water is dependent on the diameter of the pot. So when you add "drainage material" to the bottom of the pot, all you're really doing is reducing the size of the habitable area of soil. There are plenty of sites on the net that talk about perched water; here's one if you're interested: http://www.pwk.resteddoginn.ca/soilmix.php
I know it can be difficult to find a 24" pot with drainage at retail outlets. However, if you go to a nursery, they'll probably give you one free, because they commonly take plants out of that size for planting into the landscape. (These are called grow pots, by the way.) So take your grow pot, and find a pretty decorative pot that it will fit inside of; then you can put your hibiscus into the grow pot, and put that into the pretty container if you want. You can put a large plastic liner, or piece of heavy plastic, in the bottom of the container if you want to catch water if you're worried about moisture leaking through the decorative container.
If it were my tree, and I wanted to bring it in for the winter, I would root prune first so it would fit a 21" or 18" pot, or even a 14". Then I could also prune back the branches by a proportional amount; this would result in a healthier plant in the spring. Root pruning is a common practice for bonsai growers - it's one of the main things they do that allows them to keep a naturally large tree in a tiny pot. Here's a little video that shows you a basic approach to this project: http://www.finegardening.com/video-root-pruning-container-plants
If you want to save your plant, I really recommend you get it into a pot with drainage. While we're on the subject, another reason for drainage is to allow the nutrient-filled water (when you fertilize) to drain through, rather than unused fertilizer to remain in the soil with the roots. This unused fertilizer, over the course of a year, accumulates in the soil as salts, and eventually begins to damage the plant.
So I hope some of this long ramble is useful to you. Best of luck.