AFAIK that is the very reason the adaway zip exists.
From this thread's OP:
"Flash for systemless hosts file
This flashable zip preps your device so that AdAway can remain pointed to /system/etc/hosts yet never actually touch your /system partition.
A prerequisite for using this is having one of the SuperSu systemless root installed.
Download: AdAway_systemless_hosts_v2.zip :: shasum = 1bbd9ce73adb0a433995e14c4642e75ae03a9bef"
.. but then I dont use systemless and am just a test driver "hey what's that noisy thing under the hood of this car"?
My point was that this zip won't help users who have issues writing to /system/etc/hosts because (as discussed above) "systemless" root does not preclude one from mounting /system r/w. However, I admit that I don't know how the bind mount affects the write permissions of /system/etc/hosts -- I suppose that it's possible that with that in place /system doesn't need to be mounted r/w (just /su) which would sidestep a user's issue if their issue is mounting /system r/w.
Really my point is that if a user says that their adaway is having issues writing the updated hosts file then "you need to flash the system-less zip if you have system-less root" isn't actually valid advice. More helpful advice / investigation would be to figure out if that user is capable of mounting /system r/w in general.