I feel shame! And sadness! And general disappointment! Totally scoped camp before heading out as I always do a sweep, but the filter was hanging away from my tent area, on the far side of the snag, and I was focused on scanning the ground.
I was camped in a very well-hidden, but obviously previously used campsite between the trail and lake, just a few minutes before the Ruwau junction, in a foresty area right near a meadowy area. The bag was hanging on a small snag located adjacent to a small campsite with flat rock "tables" that is adjacent to 3 larger campsites. Here's the approximate location (green icon):
https://caltopo.com/m/8C35
I won't be back in the area for 3 weeks - and by then, I'm sure some lucky person will have already gone home with an almost-brand-new 6L gravity filter setup

Sigh....