How can I reuse physical memory in the absence of swapping?
You can't... one the memory is allocated, other process can't reuse it because... well, it is use!
if you dont have swap (probably because there is no HD, want to avoid burning the ssd/SD card write cycles, etc), you can try to use the zram.
This way you burn a little of ram to have a compressed swap that can save you lots of ram. It is available in kernel 3.11 and safe to use. Older kernels may have it in staging, but might still have some bugs.