Since Voikko is of no use to us, we would need either a complete words list, or info on how Finnish spelling works. Unfortunately, there is not much info about that in non-Finnish sources.
I do have a Finnish words list that cold be start, based on Wiktionary as well as Finnish words counts. But we would need a good editor to decide what is really Finnish, because there seem to be a lot of dialects and influences.
There is also the plug-in for Mozilla, which is a hunspell version, though the author claims compound words are impossible to do (also claimed by Voikko, but I sincerely doubt that; it is a challenge, but there is enough in Hunspell to make it work…)