The best way Imo, is to set up quick WordPress blogs with some content on your expired domains. Its pretty easy to get a cheap reliable hosting service. Once you set up your expired domain as a blog, create small banner ads, linking your expired domain to your parked site. I mean, my logic is that a parked page with the right Kws on a Yahoo feed makes more RPC than adsense ads. And, parking companies don't have any issues with backlink traffic. Your banner ad in essence is a hard backlink to your parked page.
For example, if your expired domain was themed on used cars, you can set up a 1-2 page blog for your expired domain and at the same time buy another domain for used car sales and have it parked. Now on the right sidebar of your blog, next to your articles, you can create a vertical banner ad which might say something like "Looking for the best deals on used cars? Find hot deals now!". Your expired domain visitor clicks on your ad, which is actually a permanent backlink, and reaches your parked page.
I know this is a longer way to the end goal, but atleast you can save your expired domain rankings and traffic from SE's by doing this. Your CTR to your parked page would depend on how well your banner is made and worded. I would also retain meta description from the expired site and probably even add content on the blog from the expired domains archived pages. In essence, I would design the blog to reflect what the actual site was. This way, the SE's stay happy and even returning visitors aren't surprised. Plus, any existing backlinks stay safely secure, which might not be the case if an expired domain is parked. Some webmasters do delete links to parked pages.
Good luck PowerUp.