I don't know if you missed it in the conversation but could it have been poo in the digestive tract? (Pushed close to the surface because she was very gravid.) They're not visible since she's made an egg sac and there was a poo like mass in her web this morning. I'll be checking to see if the squiggles return when she gains more weight. I've never kept a spider with a fertilized egg sac before.

I did see the poo comment, but I didn't want to say anything for certain about that until I heard from more people first. Spider poo comes out in liquid form, though sometimes it has tiny little black specks in it, so I was sure that the dried-looking mass in the web was not poo, but I wasn't sure if the squiggles really were the liquid poo being visible in the malpighian tubules or something. But... I got an answer!

The squiggles are actually the tubuliform silk glands! Apparently, in mature females they look that way, and it's common that the glands are pigmented. But it's not the liquid silk itself that is colored, it's just the actual gland. So far three people all agreed that's what they are: Dr. Joel Harp (Vanderbilt Univ.), Dr. Cheryl Hayashi (UC Riverside), and Dr. Geoff Oxford (Univ. of York). I think we have a winner!

I've never seen them before in any females I've kept, so maybe it just depends on how compressed against the surface of the abdomen they are, and how see-through the cuticle is. Dr. Oxford said he has seen them like this in his studies on Theridion grallator. That's a relatively see-through species, so that must be why.
I'm glad you shared these, I definitely learned something new! =D