I was hoping someone would have known something about this camp.  I will continue to search to see if I can find anything.  It seems like there should be something documented somewhere.

Just to close the loop a bit - I found out a couple of snippets of information about this camp.  It appears to be referenced in the 1969 book "100 Oregon Hiking Trails" by Don and Roberta Lowe:

I also got info from the Oregon Equestrian Trails group about a note in another book:

"Trail Riding Oregon" by Ruth Vanderhoof & Lyle Wilson, published 1988.

Eagle Creek Trail #501. Four miles in there is a horse camp; you will see the sign pointing up the hill. It has several picnic tables, one double corral and several tie racks. There are all looking rather tattered and worn. The setting is wonderful. Just past the trail to the horse camp, the main trail (#501) crosses a creek.

I think the point at which it crosses the creek is the Eagle Creek cutoff trail which goes up to the Old Baldy trail. If I find out any more info, I'll post it here.