Does the following meet your needs?
�� �� �� �� �� �� �� �� �� �� �� �� project: 'Magritte3' with: [
�� �� �� �� �� �� �� �� �� �� �� �� �� �� �� �� spec
�� �� �� �� �� �� �� �� �� �� �� �� �� �� �� �� �� �� �� �� className: #ConfigurationOfMagritte3;
�� �� �� �� �� �� �� �� �� �� �� �� �� �� �� �� �� �� �� �� versionString: #'stable';
�� �� �� �� �� �� �� �� �� �� �� �� �� �� �� �� �� �� �� �� repository: 'http://smalltalkhub.com/mc/Magritte/Magritte3/main/ ' ];
�� �� �� �� �� �� �� �� �� �� �� �� project: 'Magritte3Seaside'
�� �� �� �� �� �� �� �� �� �� �� �� �� �� �� �� copyFrom: 'Magritte'
�� �� �� �� �� �� �� �� �� �� �� �� �� �� �� �� with: [ spec loads: #('Seaside' ) ];
�� �� �� �� �� �� �� �� �� �� �� �� project: 'Magritte3Core'
�� �� �� �� �� �� �� �� �� �� �� �� �� �� �� �� copyFrom: 'Magritte'
�� �� �� �� �� �� �� �� �� �� �� �� �� �� �� �� with: [ spec loads: ##('Core' ) ];
I assume the "stupid workaround" is the fact that in your example you had to specify Magritte twice? #project:copyFrom:with: has been around for a long time in Metacello and was invented to solve this particular problem ... FWIW, this feature is documented in Lesson12 of "Metacello Configuration Tutorial Part 2" in the Help Browser ...