How can I conditionally wrap some HAML content in a tag?

You could use a partial.

foo.html.haml

- if i_should_link
  %a{:href => url}
    = render 'bar'
- else
  = render 'bar'

_bar.html.haml

.foo
  .block
    .of
      .code

Edit: Or you could use content for, I guess this is better because it keeps it all in the same file.

- if i_should_link
  %a{:href => url}
    = yield :foobar
- else
  = yield :foobar

- content_for :foobar do
  .foo
    .block
      .of
        .code