I don't have a source, but we can infer the following through svara (reason):
1) For centuries, normative Orthodox shuls have never had a mechitzah running down the middle of the Bimah
2) Those Orthodox poskim who in the past have permitted women to have aliyot under various circumstances were aware of the fact that the Bimah had no mechitzah
3) Therefore, the sight of and presence of a woman on the Bimah without a mechitzah did not constitute a prohibitive problem for these poskim.
We can also infer that Qeriat ha-Torah -- which represents a communal obligation incumbent on men and women to read and hear Torah -- is unlike the prayer service in which a mechitzah and physical separation is necessary.
The idea of running a mechitzah down the middle of the Bimah is, AFAIK, a new and innovative idea. I believe it is unnecessary for Qeriat ha-Torah, though necessary if the hazzan or hazzanit is leading the prayer service from the Bimah.
I will ask around if there are any corroborating sources...