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Would it be possible to incorporate a ping into the remailer? Say give it a list of known remailers for the chain and have it pick one at random, then ping it. If the remailer responds then send out the mail, otherwise pick a new remailer, and repeat....
One possible problem is that the final destination must then be
available to every remailer.
Say I construct an appropriate header consisting of nested
encryptions, and I use it to send a letter from
me -> A -> B -> C -> destination
where A, B, C are the remailers I chose. Say that there are a total
of 10 remailers in the universe that are known.
Here, A knows me and B, B knows A and C, and C knows B and
destination.
However, if A chooses a random remailer from [A-J], then it must
somehow make available the final destination to all of the remailers,
since there is a chance that the next hop will be destination. Now
instead of C knowing the final destination, [A-J] do.
Even if I made C the "final" hop (and it will then forward to
destination), then I have to make C known to all the remailers, where
before only B knew C.
Besides, some people may have set up anonymous remailers that aren't
public or haven't been announced.