While I enjoy using my Macbook at home, we have no Apple gear here in our office, so I was unable to do any tests in our lab environment. That's when I decided to call on the power of Google for help, and stumbled across the following Citrix article: Printers Defined on a Mac Client Fail to Auto-Create when Connecting to XenApp 5.0 on Windows 2008
Basically, the issue was that a printer driver (the HP Color Laserjet 4500 PS for XenApp 5, or the HP Color Laser Jet 2800 Series PS for XenApp 6) was no longer installed on our XenApp servers. Early on, before we learned about the pains of printing in Citrix, we freely installed drivers for printers. After finding out that was a very bad idea, I went on a driver purging spree, only allowing battle-tested drivers that were absolutely necessary to be installed on our farm. I must have deleted the required driver in this process, as it is apparently installed by XenApp automatically.
For those curious, the reason why you need these HP drivers in the first place is because, for the time being, the driver that Citrix uses for their Universal Print Driver is not compatible with Macs. So -- if you find that your Mac printers are not auto-creating, double check to make sure the required driver is installed!