I have a custom field that contains links in them and I want to auto add nofollow to those links in that custom field only. How can I do this?

Example custom field value:

    <a href="http://www.uploadable.ch/file/PnvbZ2k8B4ku/[Raizel]_Medaka_Box_01v2_(BD_720p_AAC)_(1A92BFA4).mkv">[Raizel]_Medaka_Box_01v2_(BD_720p_AAC)_(1A92BFA4).mkv</a>

I found a tutorial that auto adds nofollow to externals link
http://smartwebworker.com/485-automatic-nofollow-external-links-wordpress/
but it only applies to post content and not custom field. Anyone know a way to make it work with custom fields?