Alternate proofs of Immerman-Szelepcsenyi theorem Immerman and Szelepcsenyi independently proved that $NL=coNL$. Using their technique of inductive counting, Borodin et al proved that $SAC^i$ is closed under complementation, for $i > 0$. Prior to Reingold's theorem ($SL=L$), Nisan and Ta-Shma proved $SL=coSL$, using logspace uniform projection reductions. A 1996 paper of Alvarez and Greenlaw states "A proof of $NL=coNL$ using techniques similar to Nisan and Ta-Shma's has not been achieved although such a proof would be very interesting". I am wondering if such a proof is found in the last 14 years. Are there any other alternate proofs of $NL=coNL$ ?
Since we don't seem to have any answers, can I make a comment?

Suppose we are given $n$ bits, $X=x_1,\cdots,x_n$ and we have to complement each bit to get $\neg x_1,\cdots, \neg x_n.$ The only constraint is that the circuit that does so should be monotone. We obviously need some additional information to do this and here is one such. 

Suppose $k$ is the number of ones in the input and somehow we have this as advice. Then it is easy to see that $\neg x_i = Th^{n-1}_{k}(X-x_i)$ (i.e., on all inputs except $x_i$). Of course, the construction is monotone.

With this construction, the motivation for inductive counting is clear (at least to me). It is worth asking what other advice would work? I don't know of any other. But this may hold the key to your question.