Design rules for DNA linkers
You might want to pick something that has little or no homology with anything that exists in any organism in the databases, just in case.  

Gut feeling says to go for balanced oligos in the middle between AT-rich and GC-rich, and not have repeats or homopolymers.  If you go for AT-rich, then your complementary will be GC-rich, and vice versa. I'm pretty sure there would then be a differential in the mechanical properties of the two oligos - maybe this would be something useful to have for other reasons though?

It seems like most of the design principles might be constraints (do nots) rather then goals (dos), so you could do worse than have a generator churn out random sequences, and score each one against the various constraints.  Random seems OK, if you are then checking each randomly chosen pair against some strong rules.  Then pick ten or so pairs that all score well, get them synthesised and see whether any work...