Hey twobit - welcome to SN dude.
I've done that run several times now on various races and returns. Here's a link to some of the write-ups (including the rigs issues):
(Scroll down toward the bottom of the page.)
An overnight from Matagorda to Galveston should be no problem. We did the run from Port A to Galveston in about 30 hours.
The unlit rigs are definitely out there - and there are quite a few of them. For the most part, the charts seems to do a good job point out the rigs - but I trusted the radar we had much more than the charts during those dark nights.
Only having two on watch...all night long...while squinting in the darkness for dark steel will be a very long night. If you can work out a schedule to minimize that darkness...do it. AND use the moon as much as you can.
It's a great run. Just make sure to stay alert.