Wow, bat400, you weren't kidding about the Mound showing up clearly in the "Parcels" view at the El Portal Village site! Thanks for that.
http://flyvapnet.com/ElPortalBurialMound/ElPortalBurialMoundParcelView.gif
Now that I think about it, I'm sure you're right: The little grate must be what's left of the 'cave' entrance. It's right where I recall the 'cave' being located, but back in 1956-65 the entrance seemed - from what I recall - to be much larger; but, of course, it was probably altered subsequently for those public-safety reasons you mentioned.
Thanks for your help with all this. (It must be nice to know what you're doing when looking for and studying sites.) I realize you've squillions of other sites to keep track of here, but please don't hesitate to pass along analyses or assistance. I need all the help I can get!
Something is not right. This message is just to keep things from messing up down the road