There has been some discussion about this in the forum:
http://forum.dokuwiki.org/thread/2486 - in general partial locking is quite complex because section edits are based on the position in the code and the form data does always contain the whole content as hidden inputs, the other parts just aren't displayed. I think a better / less complex solution would be to add an automatic merging algorithm that is able to merge these edits. With this we could perhaps even think about making locks optional/soft so even when a lock exists the page can still be edited. If there is someone who wants to implement this I think this might be a nice (possibly optional) feature.