the network for radical unschooling families
Do you mean a program that unschoolers can apply for, or one that will allow them to live in another country without going to school? Unschoolers can apply to any of the existing programs, as homeschoolers. My daughter did the NSLI-Y program in Russia for 2009-2010. She had to apply and interview for it and it is a competitive program, but if you're selected then all expenses are paid (through the US State Department). The Congress-Bundestag program is similar - free if you're selected.
NSLI-Y has summer programs as well as school year ones, and if you're selected for a summer program you wouldn't attend regular school, but you would attend language classes, since that's what the program is about. During the school year the kids attend both regular school classes (in the target language) and language classes. High school in Russia was really different from US high school. My kid was fine with it - learned a lot of Russian, enjoyed some of the classes, slept through or skipped other classes - but YMMV.
You could also sign up for any of the regular exchange programs and pay for it... again, the ones I know of involve attending local schools during the school year, but summer programs are more flexible although there generally are structured activities, either language classes or some sort of work or tour-type stuff.
If you wanted a school year exchange that didn't involve school I would think you'd need to set that up yourself, by networking and finding a willing host family...