Oh I found something in ExtUtils::MM_Unix:

Not all of the methods below are overridable in a
Makefile.PL. Overridable methods are marked as (o). All methods are
overridable by a platform specific MM_*.pm file.
[download]

So, for example sub MY::c_o { "hello" } works but sub MY::all_target{ ... } does not.

I ended up using all_targets(o) like this (note that I am using the 2nd method you mentioned, i.e. overriding by creating the MY package explicitly because I want access to the SUPER:

...
WriteMakefile(%WriteMakefileArgs);

package MY;

sub top_targets {
    my $self = shift;

    my $orig_all = $self->SUPER::top_targets();
    # translations target at the end here:
    $orig_all =~ s/\blinkext\b/linkext translations/;
    return $orig_all
}
[download]

This works but I am absolutely not sure I have the order of the targets right.