Howto transplant changes of a certain changeset only for one file?

[Marko Käning]

I have two branches and want to merge in some changes made on one branch 
into the other.

The transplant extension does in principle what I want, but would - of 
course - transplant all changes of a specific changeset.

What I want to achieve is that only changes made to ONE specific file are 
carried out. So this won't be a fully transplanted changeset, rather a 
subset of its changes.

How do I achieve this in a clean way with hg?