Mike,
If I understand correctly, when your users try to find items (for requisitions) by (commodity) codes itself, which is a case on non-catalog requisition, the UNSPSC codes are coming up in their "raw" form. If it is different from what I understood, please ignore my inputs.
If the first objective is to "reduce" the number of codes that come up on search, before they could be translated to Catalogs or left as such (for non-catalog requisitions), the "raw" UNSPSC codes need to be translated to "Common Commodity Codes" (abbreviated as ccc). For this to work, your application needs to be configured via Parameters.table to use "ccc" as the SystemCommodityCodeDomainName. Multiple, and of similar category, UNSPSC codes can be mapped to a single "ccc" which would have a short better (text) description of the category. This will reduce significantly the 20,000 UNSPSC codes to even sub 10000 Common Commodity Codes. In fact, you need not even use all the 20,000 UNSPSC codes in the mappings and could use only the ones your organization needs, and hence the final number of commodity codes could be less than 5000, which are what the users are going to see on the UI.
My suggestion is slightly different from that of Brian's in that this is a step prior to his steps. I hope I'm not repeating what your orgnaization has done already. If so, again please ignore.
- Shashi