Thank you for pointing out what the problem is (I signed up for these forums just to post this note).
Since CFHTTPReadStreamSetProxy most likely doesn't do anything useful, and since it has no expected return value, I got an old project going again (temporarily) by using a Hex Editor. In both Impressario.xtra/Contents/MacOS//Impressario and Impressario2Pages.xtra/Contents/MacOS//Impressario2Pages, I overwrite the two occurrences each of the string "_CFCFHTTPReadStreamSetProxy\0" with "_CFReadStreamGetError\0oxy\0" (the \0 are the string terminators, ASCII 00). This allowed the xtras to load, and I only give up a proxy functionality that in my case isn't used anyway. You can probably pick any other call in the Core Foundation framework, the GetError seemed to work for me so I didn't look for a better match of return value and function name length.