This type of seal (Norbert Brueggen sells it over here) is not very forgiving on play.
if you can eliminate the play by e.g. bending/tweaking the pushrods then they are very good.
the solution you propose will be ok I guess, but it is just as easy to go with a bit of pipe on the dry side of the wtc
so that that pipe takes the sideways movement.
for a quick and dirty solution you could try to find a cable socket and simply mount over the seal and onto the pushrod.
graupner sells them, but they seem to be in use for bicycle and moped cabling too.
see http://www.quartel.nl/webshop/productde ... ?id=GR3356
for an example
another quick and dirty option is to try to limit the play on the dry side of the seal. I have no experience with it but it might be enough just to roll up a bit of (greased?) paper and slide it inbetween the seal and the pushrod (on the dry side).
the last resort is to tighen the nut a bit more so the O-ring grips the pushrod more aggressively (and with larger friction)
you might have done so already and therefore worn out the o-ring to become a ellipse-ring.
In any case have fun at the regatta !