I'm trying to get vi-editing mode support in macirb, but it looks this is not going to work without custom build of MacRuby with GNU readline?

If you want to avoid building macruby with rl support could try rlwrap in conjunction with macirb (http://utopia.knoware.nl/~hlub/rlwrap/). You can install it through brew, for example. Afterwards, you simply run "rlwrap macirb" and the the console will have full readline support. Hope that helps, Sven