[rebl]

@igi
I've edited the file inside the zip to:

 options = {
        OptionRecommendation(name='wordconv_exe_path', recommended_value='c:\Program Files (x86)\Microsoft Office\Office12\Wordconv.exe', 
            help=_('test tooltip ')),
        OptionRecommendation(name='docx_no_cover', recommended_value=False,
            help=_('Normally, if a large image is present at the start of the document that looks like a cover, '
                   'it will be removed from the document and used as the cover for created ebook. This option '
                   'turns off that behavior.')),

    }

I know, I've also edited to tooltip text, just to see it working (and it works).
But I still had the same error:

Python function terminated unexpectedly
  Not found c:\Program Files\Microsoft Office\Office12\Wordconv.exe (Error Code: 1)
Traceback (most recent call last):
  File "site.py", line 132, in main
  File "site.py", line 109, in run_entry_point
  File "site-packages\calibre\utils\ipc\worker.py", line 195, in main
  File "site-packages\calibre\gui2\convert\gui_conversion.py", line 31, in gui_convert_override
  File "site-packages\calibre\gui2\convert\gui_conversion.py", line 25, in gui_convert
  File "site-packages\calibre\ebooks\conversion\plumber.py", line 1041, in run
  File "site-packages\calibre\customize\conversion.py", line 241, in __call__
  File "calibre_plugins.doc_input.__init__", line 51, in convert
ValueError: Not found c:\Program Files\Microsoft Office\Office12\Wordconv.exe

What I've noticed is that __init__.py uses the slashback separator '\' in the path while the doc_input.py uses '/'. Also, one was using capital C for drive letter, while the other lowercase c. Not sure if this is relevant.

Then I've tried once more this time with Office14. I've edited
__init__.py
to contain:

 OptionRecommendation(name='wordconv_exe_path', recommended_value='C:\Program Files (x86)\Microsoft Office\Office14\Wordconv.exe',

Then doc_input.py to contain:

u'wordconv_exe_path' : u'C:/Program Files (x86)/Microsoft Office/Office14/Wordconv.exe'

Now the Preferences -> Input dialogue shows:
C:/Program Files (x86)/Microsoft Office/Office14/Wordconv.exe

Converting one single book from doc works (and the path shown in the single book convert dialogue is correct so I don't need to change it).
Trying to bulk convert two books raises the old path error.
Trying to convert only the second book presents me with a surprise. The path in the single conversion dialogue is (again !?):
c:\Program Files\Microsoft Office\Office12\Wordconv.exe
With lowercase c:
Yet, the tooltip displays my text (tooltip test).
I don't understand where this path comes from. I've edited both files and used the uppercase C:\ (if that matters), x86 and Office14.

If i manually edit doc_input.py and capitalize OFFICE14 (C:/Program Files (x86)/Microsoft Office/OFFICE14/Wordconv.exe) it appears this way in Preferences-> Input -> Doc input.
If i manually edit __init__.py and capitalize the path with different case (e.g. MiCrOsOfT), it does not appear anywhere in calibre.
Also please note that your example uses "/" in __init__.py while initially it had "\".
Then I've tried in __init__.py both with / and \ but it doesn't seem to matter. For some books the path in the single convert dialogue appears:
C:/Program Files (x86)/Microsoft Office/Office14/Wordconv.exe
For others appears:
c:\Program Files\Microsoft Office\Office12\Wordconv.exe
(Note the lowercase c
I've restarted calibre after every change with no effect.
When doing a bulk convert, the plugin will use the paths listed above, so for first book will work, second will raise the same error:

Python function terminated unexpectedly
Not found c:\Program Files\Microsoft Office\Office12\Wordconv.exe (Error Code: 1)
Traceback (most recent call last):
File "site.py", line 132, in main
File "site.py", line 109, in run_entry_point
File "site-packages\calibre\utils\ipc\worker.py", line 195, in main
File "site-packages\calibre\gui2\convert\gui_conversion.py", line 31, in gui_convert_override
File "site-packages\calibre\gui2\convert\gui_conversion.py", line 25, in gui_convert
File "site-packages\calibre\ebooks\conversion\plumber.py", line 1041, in run
File "site-packages\calibre\customize\conversion.py", line 241, in __call__
File "calibre_plugins.doc_input.__init__", line 51, in convert
ValueError: Not found c:\Program Files\Microsoft Office\Office12\Wordconv.exe