It's a bit embarrassing, I feel as if I should know this --

I was having trouble getting my solvent to come back out of solution, probably because of its low volume relative to the mud. I had seen it suggested that the addition of saltwater can help.

Okay, say I did that. Now say somehow I ended up adding a lot of saltwater.

Obviously the dilution will affect the pH. Will the salt itself also do so, enough to interfere with extraction?

A way you can tell if you do not have the optimal pH without having an actual pH meter is relatively simple when extracting the way you are.

For example: if when you add your solvent to your basic solution it does not immediately begin to separate, or if it doesn't separate at all over time, it is a clear indication you need to add a bit more basic solution.

Since you're describing your solution as "mud" I'm guessing that it's pretty thick and that you did an STB with pre powdered bark.
Often you need to use as much as twice the water called for in an STB to get the solution free flowing enough that the naptha doesn't get trapped.
Ideally, this happens before the naptha is added because it can be very difficult to break back out once it's stuck in there.
You did exactly the right thing though - adding more water. The salt might help, it certainly won't hurt. Don't worry about the pH, if you used the lye called for in a typical STB, it's fine.
If it stays stubborn after adding enough water that the solution is free flowing, adding some more naptha and straining the whole mess through an old t shirt a couple of times can help.