Hey moongrad,
How I would support this kind of cake is using straws in the bottom tier the exact height if the cake. Make sure the top tier is sitting on a board and place that on the bottom tier. Then use straws the height of the top tier to support the topper. The number of straws will depend on how heavy the topper is.
Another alternative if you have time is buy a support system that your customer would return to you. I love using this system and have never gone back to straws. It is so easy to use and I never worry about the cake falling or squishing. http://www.stressfreecakes.com
