Fred Lunnon writes:

I haven't understood how this linkage is intended to function

Start with a bar of length 2 free to pivot around its (fixed) center; call the center 0 and the endpoints +u and -u. Now add a bar of length 2 centered at (and free to pivot around) +u and another bar of length 2 centered at (and free to pivot around) -u; call the endpoints of these bars +u+v, +u-v, -u+v, and -u-v. Add a bar of length 2 joining +u+v and -u+v and another bar of length 2 joining -u+v and -u-v, to ensure that the names we've given these points are truly apt. Now add four more bars of length 2 centered at +u+v, +u-v, -u+v, and -u-v; call the endpoints of these bars +u+v+w, +u+v-w, +u-v+w, +u-v-w, -u+v+w, -u+v-w, -u-v+w, and -u-v-w, and add eight more bars of length 2 joining +u+v+w to +u-v+w, etc.

but have you considered a simulation in Mathematica or Maple; or more satisfactorily, Java / Javaview or GeomView?

I hadn't considered this (since I don't have the relevant programming skills), but it's a good idea, and if anyone implements this please let me know! Jim