A couple points: you can make calls to the WlanControllerDetails resource to get pages of info per request rather than one controller per request (for example, /webacs/api/v1/data/WlanControllerDetails?.full=true&.maxResults=500).
We are aware of this limitation and how it's cumbersome (see CSCvb73710). The reason we don't show UDIs for WLCs in the InventoryDetails resource is due to performance and historical reasons. Serial numbers for WLCs are not stored in the same place as they are for routers and switches; historically, this is due to the fact that WLC management functionality came from Cisco Wireless Control System (WCS) and functionality for wired device management came from Cisco Network Control System (NCS). The two products modeled some information differently, and those differences still exist in parts of Prime Infrastructure. We could change the InventoryDetails resource to pull data from the two different sources, but when we've experimented with this in our large-scale test environments the response time was severely degraded.
The good news is that the team responsible for wireless device inventory is taking up work currently targeted for the next major release of Prime Infrastructure (3.2) that will unify these disparate models, and allow us to report UDIs for WLCs via the InventoryDetails resource without negatively impacting performance.
