@acrylian - yes, it's customized and working well in my [i]official[/i] website http://www.indirizzofantasma.net/zenphoto/. I might come back to this issue in a separate thread, thankyou.

@sbillard - yes you can consider the (so long!) topic closed, I've solved the issue by going back to the official build, 'cause the nightlybuild is IMHO not dealing well with the embedded keywords, while the official works perfectly.
but that's another issue, as I said above.

@anyone - who dares reading this thread and is interested, this is how I solved the issue in few simple steps:

1. reinstalled another zenphoto 1.4.1.6 [8326] http://www.indirizzofantasma.net/test2/
   all settings and custom theme were replicated from my working site.
2. created an album containing the same 2 testing photos used for testing 'till now. one has XMP datas, the other only IPTC
   e.g. album: [i]test pre_mod[/i]
   note that I've added via admin a tag "selected" to one file. both the embedded keyword "finestrata" and the zp generated "selected" where showing in sidebar and in the info table beside the image, together with all metadata [i]except[/i] the one I want to retrieve
3. via myPhpAdmin added 2 fields to the image table, right after IPTCLocationName:
   `

IPTCContentLocationCode varchar(3) ...

IPTCContentLocationName varchar(64) ...

4. opened zp-core/class-image.php and found 'ContentLocCode' => '2#026',

'ContentLocName' => '2#027',

changed to 'ContentLocationCode' => '2#026',

'ContentLocationName' => '2#027',

5. opened zp-core/functions.php and foundstatic function setexifvars() added the following 2 lines to the array, where it maked sense: 'IPTCContentLocationCode' => array('IPTC', 'ContentLocationCode', gettext('Content Location Code'), false, 3, true),

        'IPTCContentLocationName'   => array('IPTC',     'ContentLocationName',     gettext('Content Location Name'),                                           false,          64,     true),

`
6. created via admin another album, exactly the same as the previous one, except no "selected" tag was added
e.g. album: [i]test post_mod[/i]
7. done!
the files uploaded [i]before[/i] the above described mod didn't show the wanted field, but those uploaded [i]after[/i] where finally showing that field
e.g. Cannaregio
8. as a next step, I've used the [i]refresh metadata[/i] admin tool.
the result is that the tag "selected" was cleaned out, while the Content Location Name appeared also in the [i]test pre_mod[/i] files.

so after 2 weeks and a lot of time spent on it I know that I only need to re-assign the "selected" tag to a 100 photos in my "official" website...
;-)

Note that the current release is 1.4.2.3….

I prefere to not upgrade now, as something wasn't working as I expected with the nightly build (e.g. loosing embedded keywords).
FYI I've refreshed the metadata in my "official" website.
I was expecting to loose the [i]selected[/i] tag for 101 photos. note that this tag wasn't embedded as keyword but assigned via zenphoto admin.
misteriously "only" 49 photos had the [i]selected[/i] tag washed out.
after some investigating I came to this conclusion:
[i]if[/i]
there was some keyword embedded [i]and[/i] the [i]selected[/i] tag: the [i]selected[/i] tag have been erased when I refreshed the metadata
otherwise
the [i]selected[/i] tag was mantained in its place.
hope this can help.