I reviewed all my merges and discovered I'd lost changes in:
PRPierFrame>>initialRequest
PRViewCommand>>initialRequest
MAOneToManyComponent>>add

With the corrections in place, Pier appears to working once again. There are a couple of anomalies - the Pier logo doesn't appear on the home screen and I received one walkback while clicking around. I'll investigate these problems later. I've also yet to look at any add-ons including the Pier-Blog.

I've loaded the code into a fresh seaside3.0a5 image (with updated Gofer) and the code loads cleanly.

For reference the load script I used was:

"Load Magritte for Seaside 3"
Gofer new
renggli: 'magritte2';

package: 'Magritte-Model';
package: 'Magritte-Pharo-Model';
package: 'Magritte-Tests-Model';
package: 'Magritte-Tests-Pharo-Model';
package: 'Magritte-Seaside';

package: 'Magritte-Pharo-Seaside';
package: 'Magritte-Morph';
load.

"Load Pier for Seaside 3"
Gofer new
renggli: 'pier2';
package: 'Pier-Core';
package: 'Pier-Pharo-Core';

package: 'Pier-Model';
package: 'Pier-Pharo-Model';
package: 'Pier-Tests-Model';
package: 'Pier-Seaside';
package: 'Pier-Pharo-Seaside';

package: 'Pier-Security';
package: 'Pier-Pharo-Persistency';
package: 'Pier-Tests-Security';
load.

PRPierFrame registerAsApplication: 'pier' kernel: (PRKernel new root: (PRPage new title: 'test'; contents: 'Pier on Seaside 3.0'))