It is possible to use functionalities from custom dlls in your iLogic rules, you just need to declare the AddReference “adn.dll” (replaced by the name of your dll obviously) at the top of your rule.
iLogic will look in the following places to find your dll:
1) The folder of the document running the rule
2) The workspace path of the current project
3) The library paths of the current project
4) The external rule directories set in Tools >> Options >> iLogic Configuration >> External Rule Directories
5) The reference path set in Tools >> Options >> iLogic Configuration >> iLogic Addin DLLs Directory
In all these locations, it only looks in the directory itself. It doesn’t go into subdirectories.