I understand what you mean by a large majority of your customers that are on shared hosting enviroments, even then most have aspell installed and a quick inquiry to the host could provide them with all their information they need.
Why not look at how FCKEditor does it? They provide the option for aspell (Speller Pages), ieSpell, or WSC (http://www.spellchecker.net/)'s hosted version.
That way you could give your board users an option. Just a configuration setting in the admin that they can choose the WSC option (which is like SpellerPages but has ads on it), aspell (which they would need to know the path to), or ieSpell (which you guys have already)?
Furthermore that perplexes me is that you guys have a "spelling" folder already in place with plenty of php scripts, is this just a testing folder or is there some way of using it? If all it checks for is ieSpell then why even have the "spelling" folder in there in the first place with the php scripting?
I looked at phpspell (http://mods.invisionize.com/db/index.php/f/5373) but it's only up to 2.1.7. Which I even tried to implement and it failed miserably.
Thanks,
Chad