I had donuts so I actually purchased seeds so that I could drop at least one in each neighbors town as it becomes eligible and I personally don't expect reciprocation.
I don't think you can drop a seed if the neighbor doesn't have mutant corn for you to tap, At least I don't see an icon.
Strategically, It seems like you would want to place the seeds in a neighbors town where the owner will find the mutant seed quickly. I don't know if it was correct but I read in another thread that the seeds were "time sensitive" and would disappear in a while if not found, the sooner found the higher reward for both parties. If that's the case it may be a reason why a neighbor would drop a bunch at one time in an active town. I wouldn't feel guilty about not being able to return the favor if you can't. :)