This certainly isn't an awesome solution but couldn't the GC backtrace(3) the current process and look at %eax at all C stack frames to additionally include that value in the "pointers currently plausibly in flight" list?
The correct way to handle this is to add the object reference to the GC's "root" set while you're using its guts, and removing it again when you're done.
Another possible solution is to allocate the string object and its character representation in one chunk of memory. This only works for immutable strings which never share substructure, though. The reason this works is that most conservative GCs will consider objects live as long as there is a pointer pointing to somewhere within a chunk of memory, not necessarily at the beginning.
 note: I'm not a Ruby coder but I fixed a very similar problem in a Lua implementation about 4 years ago. That one wasn't even conservative GC. EDIT: I told the story of that bug on HN 3 years (!) ago http://news.ycombinator.com/item?id=217189
 worse, it probably doesn't blow up immediately and instead causes memory corruption.