For background, see: Using the model debugger to change the model itself.

To find all ViewModels with a specific tag:

Span.allInstances->select(s | s.TaggedValue->select(tv | tv.Tag.toUpper='MVC')->notEmpty)

To add the tag MVC to all ViewModels that don't already have it:

Span.allInstances->select(s | s.TaggedValue->select(tv | tv.Tag.toUpper='MVC')->isEmpty)->forEach(s |
let newtag = TaggedValue.Create in
  (
    newtag.Tag := 'MVC';
    newtag.Value := 'True';
    newtag.ModelElement := s
  )
)

To add the tag Eco.BusinessDeleteRule to all association ends that don't already have it, set it to NeedNotBeEmptyNoWarning:

AssociationEnd.allInstances->select(s | s.TaggedValue->select(tv | tv.Tag='Eco.BusinessDeleteRule')->isEmpty)->forEach(s | 
let newtag = TaggedValue.Create in 
  (
    newtag.Tag := 'Eco.BusinessDeleteRule';
    newtag.Value := 'NeedNotBeEmptyNoWarning';
    newtag.ModelElement := s
  )
)