To the original question, a custom rod can be light if the builder strives for it. I've seen Orvis blanks built up to be lighter than their factory counter parts by using thin coatings, and cork reel seats with light weight bands.
I'd venture that most custom builds are heavier than their factory counter parts - using a slightly bigger handle, building a rod with "burl", "exotic" or other composite corks, adding feather inlays, using nickle silver rather than aluminum, and doing long signature wraps will all bump up the weight. I'm not saying that these techniques and components make a bad rod, not in the slightest, but they're things you want to stay away from if you're primary focus is grams.
The key is to discuss with your builder what you want. If you ask them to keep it light they should very well be able to.