EDLB, non-complex crystal oscillators are in the +/- 50 to +/- 100 parts per million tolerance range, typically. They tend to be fragile for shocks. And they chirp significantly as the oscillator circuit turns on. (It would have to power cycle to save energy.)
I am not sure what the medium would do to long pings with multipath and otehr effects.
ACLS65, I have long ruminated on the concept of ping back when pinged. That, however, requires a complex receiver to avoid false triggering. That means more things to fail even if it's square mm of silicon surface. And it continuously sucks power without producing a useful output.
An RC oscillator with an RC timer for both the 1 second period and the 10ms transmit period power levels can be kept very low with most of the power used being devoted to providing an output at all times with very few parts to break or die of old age. I have not discarded the transponder concept in my mind, yet. But it doesn't seem to be as big a win as one might want or expect at first glance.
{^_^}