I've also seen people camp along the river (South Fork South Platte) just upstream of the Colorado trail bridge. Park at the bridge, walk across and upstream and you'll find some flats there near where the trail starts to climb. If you're not familiar with the area, this is part of the Deckers run and probably the closest you're going to get to your destination runs. Waterton is just downstream at the confluence of North and South Forks of the South Platte, and Foxton is up the North Fork from the confluence.
"A witty saying proves nothing."
- Voltaire (1694-1778)