After digging through SCCM's SDK Documentation, what I ended up doing was making the updates available but not mandatory. This populates a WMI class called CCM_SoftwareUpdate in the root/ccm/clientsdk namespace, see here. A simple query such as
"SELECT * FROM CCM_SOFTWAREUPDATE WHERE COMPLIANCESTATE=0 AND EVALUATIONSTATE < 2"
will get you the updates that are available that haven't been installed yet. When updates have been installed, eventually they will be removed from this namespace, but it takes some time. If you have to stop and restart installing updates for some reason, it helps to filter out the updates that are partially installed. The msdn page has more info on the different compliance states and eval states. That array of updates is then passed to the InstallUpdates method of the CCM_SoftwareUpdatesManager class in the same namespace, and you can monitor the progress of the updates individually (which is why I chose to pass my updates to that method one by one instead of all at once. Passing them in all at once doesn't really do much beyond queuing them and installing them one by one anyway, from what I've seen).
I whipped up a quick and dirty gui in C# (You can view it here), which I set as a logoff script in group policy, so when it runs the users cannot ctl alt del out of it, or press buttons to close the window either. It's as close to the real thing as I could get (we have control over the login background so I made my app's background look the same for consistency).
The sad part is we probably won't adopt that solution b/c of the multitude of laptops in our environment. No one wants to shut down their laptop and take it home for the day only to have to wait an indeterminate amount of time installing updates, which will at that time seem like forever.