Now that I've managed to knock down the number of bugs in NEWCARD to a more reasonable level, I've started looking at ExpressCard again.
As you may know, FreeBSD supports ExpressCard right now for usb devices. They all work, and there's no additional work needed here, except maybe to make more drivers. In fact, I recently purchased an ExpressCard to CardBus adapter (one that lets me plug in an expressCard into a CardBus slot) and it worked with all the usb-based ExpressCards that I have laying around with no changes.
The problem with ExpressCard for PCIe-based devices is one of resource allocation. If the BIOS allocates the resource, then the ExpressCard works without hot-plug. If the BIOS doesn't, then we can see the device with pciconf -l, as well as probe the device. However, since there's no resources setup for the PCI bridge, attempts for the driver to allocate them fail. There's also some bus numbering issues as well.
I've started to attack the resource problem. I'll keep people posted