By now I've heard from about three people about how much the GS500 had a really, REALLY shitty float needle design (if I remember correctly, the needle seat is made of plastic or rubber)

That said, and since I'm going to have my carbs out for cleaning, I ask you folks this: Is there any aftermarket fix for this?

I know you can buy a float needle assembly from Suzuki, but that just replaces broken material with the same, not-yet broken material.

Is there another needle assembly or just needle seat from either an aftermarket manufacturer or flat-out OTHER BIKE that would work?

Many many bikes never have this problem. I believe it's mainly due to bikes sitting and getting gunk in the tank or gas going bad in the carbs. 

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Its rubber into metal. Its not problem free, but its pretty good, not a big issue IMHO. The bigger issue is that the 89-00 ones drift, every 2-3 years you have to re adjust it.
Cool.
Buddha.

any hints as to how the needle seat is removed or is it better to leave it and just replace the needle?

On the 89-00 its is just pulled out. There is a O ring fitting it to the carb body. On the 01+ there is a screw in the carb body with a big washer holding that thing down. Unscrew it and the seat pulls out.
Cool.
Buddha.