blekko allows you to do this with their feature to create your own slashtags. for unscatter.com I took the approach of using other sites apis but the caveat be

blekko allows you to do this with their feature to create your own slashtags. for unscatter.com I took the approach of using other sites apis but the caveat being I never intended it to compete with Google or bing or the like as it's results are based on recency rather than the relevancy. in one incarnation of unscatter.com I did something similar to what you did except I used boss. one thing you should consider is there is a limit on how many characters are in a get request and you will hit it fast embedding all those site: declarations. good luck.