Adding an alpha channel to geoTIFF image

Trying to add an alpha (transparency) channel to the RGB data in a georeferenced tiff image. I was led through the process of creating the geotiff in a separate question. The first part of the code creates a geotiff from the attached png file, the second part creates a mask from white regions, appends it to the RGB data and attempts to modify the appropriate tags before rewriting the geotiff. However the resultant image is blank. I suspect I'm doing something wrong with the 'photometric' tag but can't see the problem.

file = 'uk_dT.png' ; 
[path,name,ext] = fileparts(file) ;

I = imread(file) ;
I = flipud(I);

lonmin = -17; lonmax = 10; latmin = 47; latmax = 63;

% Write to geotiff
R = georasterref('RasterSize',size(I),'LatitudeLimits',[latmin,latmax],'LongitudeLimits',[lonmin,lonmax]);
tiffile = strcat(name,'.tif') ;
geotiffwrite(tiffile,I,R) % original code

%%Now we have a geotiff but need transparency.

% Make alpha mask by identifying white regions
S = sum(I,3);
mask = 255*ones(size(I,1),size(I,2));
mask(S==765) = 0; % create mask
I = cat(3,I,mask); % concatenate alpha channel onto RGB data

info = geotiffinfo(tiffile); % reload RGB tiff data

% use Tiff tags to append alpha data
tiffile2 = strcat(name,'2.tif') ;
geotiffwrite(tiffile2,I,R,'GeoKeyDirectoryTag', ...
  info.GeoTIFFTags.GeoKeyDirectoryTag,'TiffTags', ...
  struct('ExtraSamples',Tiff.ExtraSamples.AssociatedAlpha, ...
  'Photometric',Tiff.Photometric.Separated));

[S Jones]

Edit: Having played with this problem a bit more, it appears it is impossible to add an alpha channel to a geoTIFF in Matlab (at least without some clever workaround) for a couple of reasons:

1: the 'Photometric' tag should be set to 'RGB', as the colour space we're creating is RGB+alpha. However there is an interlock in geotiffwrite which prevents you from writing anything other than an x*y*3 array when this tag is set to RGB. The code I utilised above (found somewhere online) gets round an error by using 'Separated' instead, but this is associated with the CMYK colour space so is surely not the right setting.

2: You set the 'ExtraSamples' tag to inform that something other than RGB data is expected. But in other examples I've seen, you also have to set the 'SamplesPerPixel' tag to 4 to reflect this. Geotiffwrite does not recognise this as a valid tag. In fact, diving into the geotiffwrite code and looking at the tags which are valid, there are only a fraction of the options available compared to the tiff() command.

I would love to be proved wrong but it appears that I'm going to have to manually load hundreds of files in an external package and change white areas to transparent!