Ever played Games Workshop's Judge Dredd, Graham? I know you were a 2000AD fan. I have a copy, but never played it, worth the money for all the old strips though.
Not to dog ont he original poster, but most of what I've heard about D20 is that it doesn't handle big robots or starships very well at all. Despite this companies like GoH and DP9 release sourcebooks, but I think thats mostly because of the larger market share. A large amount of role-players out there don't realize there's stuff other than D&D.
I dabbled with Macross designs in DP9's Silhouette system, but as this point I think if I was gonan do it I'd do it using Lightning Strike. Allows for a larger overall battle and I'm not one for overly detailed rules. Christ, I barely role dice at all when I'm GM'ing... <_<