You are from Roma, Texas, right ? And that's right on the border of Mexico. And I see , from the map, that this "sugar lake" you speak of , is on the Mexico side of the border.
And in Mexico there is no all-reaching border-to-border type laws about metal detecting (fumble fingers caliber stuff anyhow). As long as you're not on sensitive historic monuments. Thus metal detectors are a common site on all the mexican tourist beaches. I would assume the in-land lakes would be no different.
and knowing Mexico: they have bigger problems on their plate, than to dream up a "permit" for detecting at a lake! If there's a list of rules listed there , for whatever entity administers that lake . Like do they even have a 'park's dept' down there? Or is it simply maintained by the various fishing resorts I see listed as being there.
If you see nothing that says "no metal detecting" and see nothing that says "permit required", you are probably over-thinking it. If you can go there and fish and swim (as it appears you say is already popular draw), then just go.