You could use the wordlists in the 'dutch' package (wdutch and idutch binaries), which were forked off by Erick Branderhorst before the restrictive license was applied. I got the impression that Erick knows Piet personally, so he can probably clarify the situation.
It would also be beneficial if you could actually depend on that package for the words, so that they aren't installed twice :-) Erick might be interested in a patch that lets his wordlist package generate the format that aspell needs, if necessary. Richard Braakman