Beats me, though it's very close to bedtime here. I tried the following to no avail:

my $tmoe = Test::MockObject::Extends->new( $foo );

overload::OVERLOAD(
    ref $tmoe,
    '""'       => sub { "mock stringy\n" },
    'fallback' => sub { "default mock overload\n" }
);
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My guess is that the magic's not registering properly, though I have no idea why that should be. Calling ("" directly works.