It might be a bit late to answer this, but at the National Library of Luxembourg we have been using METS/ALTO for digitization projects for more than 10 years. All the technical metadata about and resulting from the digitization project is stored in MIX schema elements within a techMD section, as shown below:
<amdSec ID="IMGPARAM00004">
<techMD ID="IMGPARAM00004TECHMD">
<mdWrap MIMETYPE="text/xml" MDTYPE="NISOIMG">
<xmlData>
<mix:BasicImageParameters>
<mix:Format>
<mix:MIMEType>image/tiff</mix:MIMEType>
<mix:ByteOrder>little-endian</mix:ByteOrder>
<mix:Compression>
<mix:CompressionScheme>1</mix:CompressionScheme>
</mix:Compression>
<mix:PhotometricInterpretation>
<mix:ColorSpace>1</mix:ColorSpace>
</mix:PhotometricInterpretation>
</mix:Format>
<mix:File>
<mix:ImageIdentifier imageIdentifierLocation="//Dss2/d/docworks/EXPORT/bnl_lw/1900-09-10_01/tif">1900-09-10_01-00004.tif</mix:ImageIdentifier>
<mix:FileSize>25003209</mix:FileSize>
<mix:Orientation>1</mix:Orientation>
<mix:DisplayOrientation>1</mix:DisplayOrientation>
</mix:File>
</mix:BasicImageParameters>
<mix:ImageCreation>
...
<mix:ScanningSystemCapture>
<mix:ScanningSystemHardware>
<mix:ScannerManufacturer>ASSY.SA</mix:ScannerManufacturer>
<mix:ScannerModel>
<mix:ScannerModelName>Digitizing line</mix:ScannerModelName>
<mix:ScannerModelNumber>DL 3000</mix:ScannerModelNumber>
<mix:ScannerModelSerialNo>2006 11 25</mix:ScannerModelSerialNo>
</mix:ScannerModel>
</mix:ScanningSystemHardware>
<mix:ScanningSystemSoftware>
<mix:ScanningSoftware>LIBFORMAT (c) Pierre-e Gougelet + Page improver</mix:ScanningSoftware>
<mix:ScanningSoftwareVersionNo>1.0.2519.16684</mix:ScanningSoftwareVersionNo>
</mix:ScanningSystemSoftware>
<mix:ScannerCaptureSettings>
<mix:PixelSize>0.0846666</mix:PixelSize>
<mix:PhysScanResolution>
<mix:XphysScanResolution>300</mix:XphysScanResolution>
<mix:YphysScanResolution>300</mix:YphysScanResolution>
</mix:PhysScanResolution>
</mix:ScannerCaptureSettings>
</mix:ScanningSystemCapture>
</mix:ImageCreation>
<mix:ImagingPerformanceAssessment>
...
</mix:ImagingPerformanceAssessment>
...
</mix:mix>
</xmlData>
</mdWrap>
</techMD>
</amdSec>