My company holds quarterly networking meetings at University business
schools in major cities around North America.  We bring in Sr.-level
speakers to talk about human resources or leadership issues and
provide snacks.  We try to seat about 150 per event.  We've been
holding the meetings in the morning (7:30-9), and I'm trying to
determine if it's better to hold these in the morning or after work
(5:30-7).  We have meetings in LA, Philadelphia, Columbus, Boston,
Atlanta, NY, DC, Toronto and Montreal and will be going into other big
cities soon.

So the question is 1) best time to hold such networking meetings
2) any suggestions for how to market these programs in local
communities beyond the more obvious (we're pretty good at marketing).

Hello,
I am making a couple of assumptions that may be wrong.
You hold the meetings at business schools, but the meetings are not
intended specifically for the students (you ask about marketing them
to a broader public).
"Networking" is your expression for a presentation or a podium
discussion, at which the company presents itself, its services
probably (rather than products) in a low key manner by letting senior
people discuss "human resources or leadership issues" in an
informative way that reveals their expertise and the company's
strengths, at the same time gathering feedback, maybe new customers.

If these assumptions are wrong, please explain.

Two points:
Morning meetings and evening meetings will appeal to different groups of people.
Commuters probably can't change their schedules so easily to attend a
morning meeting, whereas they could stay in town longer in the
evening.
Some people just shouldn't be talked to before 9 am.

"Networking" to me describes a meeting which allows  - or enhances - 
the opportunity for mutual, informal communication, also among the the
attenders, not just between the hosting company and them (the
situation I am assuming).
If my assumption is wrong, then an evening meeting with the
possibility for the attenders to continue their discussions, either by
allowing the meeting to continue or by their doing so after the
meeting, if they wish.

A morning meeting precludes this kind of networking.  On the other
hand, if you don't want this to happen  - uncontrolled discussion - 
then evening meetings might not be such a good idea.