Should I be able to then use pcl::PointCloud<T> as a message type for
a message_filters::Subscriber as well?

Yes! message_filters::Subscriber wraps around a ros::Subscriber, so this will just work.

If so is there code anywhere
where this is used with an ExactTime synchronizer that I could look at
as an example? I see that, e.g., pcl::fromROSMsg is still used in
sac_segmentation.cpp in pcl_ros.

I don't believe we have a code example right this moment, but Radu is hard at work converting pcl_ros to use ros::Publisher/Subscriber everywhere.

Cheers,
Patrick