This is an interesting scenario I never thought of before.
Unfortunately, there is no way to do that in Registry Finder directly. And I cannot propose any full fledged workaround. Just an idea that may or may not be useful to you.
Briefly, the idea is to load hives with "reg load" command line and search in the root key where the hives are loaded into.
The problem is that a hive can be loaded into a unique immediate subkey of HKLM or HKU.
That is, you can load the offline SOFTWARE hive into, say, HKLM\offline-software and the offline SYSTEM into HKLM\offline-system, but you cannot load them into HKLM\offline\software and HKLM\offline\system respectively.
Nevertheless you can search in all that subkeys at once by separating them with semicolon: "HKLM\offline-software;HKLM\offline-system".
That is all that RF can do right now. Fully supporting your scenario by Registry Finder would be a major feature for for I have no time to implement. And I doubt I will have it in the nearest half-year, sorry.
Sergey