There are some recipes out there for mock apple pie that use zucchini (courgette). When cooked, they very much resemble apple. Use the apple syrup, leave out the dates, use the walnut and cinnamon.

Mock Apple Cobbler

Or you could go ahead and just treat this as a feast day. Make the harroset as you have in the past. Have a small portion. Passover only happens once a year...