It should be possible to program a checkuser watchlist feature. The watchlist consists of IP addresses or ranges of interest. A hypothetical design would be like this:
* 126.96.36.199 User:ABC * 188.8.131.52/16 IP range for User:XYZ ...
When a checkuser runs a check, if any of the IP addresses returned match an item or range in his watchlist, the associated information should be displayed next to it. A hypothetical example would be the following results from check User:FooBar
184.108.40.206 (block) (1 Aug 2016, 12:00 -- 2 Aug 2016, 12:00)  (~86 of all users) 220.127.116.11 (block) (18 Jul 2016, 12:00 -- 28 Jul 2016, 12:00)  (IP range for User:XYZ)
The watchlists can be user-specific (each person with checkuser access has his own watchlist), or global (all the checkusers share the same watchlist), or both.