The helper you gave as an example is a valid helper, and has an implicit context: "thisModule". You can define any helpers in a library, even the ones that refer to types from metamodels (i.e. metaclasses).
I can imagine you'll want to define new collection operations as helpers with a collection type context, which is not possible in regular ATL. Support for this is added in ATL/EMFTVM. I have added it to the corresponding wiki page. The specific "invertList" operation you mentioned already exists in OCL 2.2 as the "reverse" operation, and is therefore part of ATL/EMFTVM as well.
Finally, experimental support for adding you own higher-order collection operations (called "iterators" in OCL) can be found in SimpleOCL, which compiles to ATL/EMFTVM bytecode and can be used from ATL. See iterators.simpleocl for an example of what you can do.