Feel free to steal anything: test PSD files, etc.

If you want to implement layer blending in PSD.rb (required if you want to be able to export layer groups as images) then check libpsd C library - I think it has the most sophisticated open-source PSD blending implementation available.

Your PSD.rb and https://github.com/layervault/psd-enginedata libraries are very nice too; unfortunately I'm busy with other projects now, but maybe somebody will take effort and port PSD.rb features to psd-tools, or I could do this in future - the LayerVault's code will be very helpful.