Well there was a project to develop a search engine to search multiple sites, Discussed in these threads
Unified Search Engine - The ball is in motion
What is a good name for a cross-publisher ebook search engine?
There is a Wiki entry as well for
Unified Ebook Search Engine and there is a Java beta version of the search engin which searches a few of the sites which have open API's called
JAM Which you could try out.
Its still pretty beta, so not that usable, but its worth keeping an eye on.
I did try to create a custom google search for those sites but could not get it to work. Perhaps someone who's can make more sense of the system could implement one until a custom search engine is ready.