Although RyanMcGreal didn't refer specifically to Glass-Steagall, it's a good bet that "re-regulating" (i.e. re-introducing measures from that law) might be a bad idea in several cases, Regulation Q being one of them.
Megan McArdle is a terrible journalist. I would suggest double-checking all of her claims.
For instance, she recently wrote that wealth inequality was probably worse in the 90s than it is now. In fact, it's now much worse than it was then. She didn't bother to check her own assumption.
Unfortunately, long experience has shown me that it's not worth trusting McArdle unless you have all of her sources right out in front of you; I believe the polite way to put it is that she "massages the facts" a bit :)
Although RyanMcGreal didn't refer specifically to Glass-Steagall, it's a good bet that "re-regulating" (i.e. re-introducing measures from that law) might be a bad idea in several cases, Regulation Q being one of them.