Most of my cookware is aluminum. I've seen induction interface disks, but they are quite expensive. Would it work to use a small cast iron griddle as an interface disk for my aluminum pots and pans?
Yes, it will work. The induction interface disks are practically the same thing as a griddle.
Of course, both disks and griddles mean that the induction stove will behave like a resistive stove (slow heating, etc.) If you want to get your money's worth from the induction stove, replace your cookware. The combination with cast iron is great, as it gives you all the advantages of cast iron without the major disadvantage of glacially slow heating of thick pans. All other induction-capable materials will work better than alu on a griddle/disk too.
Induction cooking is accomplished through a magnetic field created in the hob that excites (causes to vibrate) the IRON ATOMS in the cooking vessel you use. This is what causes your pan to heat. If there is no iron in the pan (non-ferrous stainless steel, tin lined copper pots, aluminum pots and pans, glass or clay cooking vessels, etc) they WILL NOT heat! This is why people are advised to check whether cooking vessels are "induction friendly" by testing whether a magnet will stick to it.
As a result there is a direct correlation between the iron content of any given pot or pan and how quickly it will come to temperature. This means that pure iron pans such as DeBuyer's "Mineral B" brand are the most responsive, followed closely by cast iron ware (including ceramic clad cast iron), then carbon steel, and last of all comes "induction friendly" stainless steel.
I have been cooking with induction for two years now. I invested in an induction interface disk early on. I used it only once because I found it to be too inefficient. Instead, I use my largest cast iron frying pan to accomplish heat transfer to non-ferrous cooking vessels, but be warned! You can warp pans by doing this if you use very high (relatively) temperatures. Adding an inch or two of water to the cast iron frying pan and then elevating the non-ferrous pan by setting it on spacers that allow the simmering/boiling water to circulate under it not only reduces the risk of warping but also results in less temperature fluctuation and more even heat distribution in the non-ferrous pan.
I have yet to see an induction interface disk that is made of anything except stainless steel. In my opinion, they are not satisfactory and are unreasonably overpriced. In addition, it is just so much easier to use the right pan for the right heat source. In this day of portable butane gas burners such as those Iwatani makes, as well as single unit electric hot plates, adaptive induction interface plates just don't make sense to me.