Don't get discouraged, this is nothing new, and probably won't turn out to be anything sufficient but you miss 100% of the shots you don't take so I might as well try.

Tonight I was doing some research on the LRNG (Linux Random Number Generator) and I came across this interesting document

It highlights how the LRNG works in various systems, embedded systems, and directly (but briefly) targets OpenWRT. Mind you I don't have a degree in Computer Science, nor Computer Security bla bla bla, but according to this document, the ONLY source of entropy in kernel 2.6.10 (yes it is pretty old) is from network traffic. Apparently in this version, entropy was not carried across reboots, though I believe most current networking devices that run Linux do save it now, but let's not jump to conclusions yet. I haven't finished reading the whole document as it is very late but I figured I'd share to see what you maybe more, maybe less advanced people think.

Even if this practically exists across a small fraction of routers still in use today, it certainly seems to be something worth looking into. Let me know what you think!