> the idea was to pass the FILE and extra OLP arguments to the custom 
function provided by the user

I have misunderstood what you suggested. But, I wanted to maintain 
consistency with how the `file+headline' handles function as a target 
where the lambda takes no parameter and is expected to return the 
appropriate type of value while already visiting the file.


 > You are still calling `org-capture-expand-function-or-symbol' without 
passing the rest of OLP there. Or was it the intention?

It is intentional. The only other type I allow (besides a list of 
headings) is a single function, a single bound symbol and `nil'.


 > Also, is there any point calling `find-file-noselect' when the outline
 > path spec is _not_ a function?

No. I'll fix that. I ended up removing 
`org-capture-expand-function-or-symbol'in favour of explicitly testing 
for function and bound-symbol to allow more fine-grained use of 
`find-file-noselect'.


 > Finally, you need to describe the calling convention and environment
 > when the function spec for OLP is called - that it is called with no (or
 > some?) arguments and that the current buffer is the FILE-visiting buffer.

Sounds good.


I've attached the updated the patch.