If you can, installing your own version of SpamAssassin or something like that will probably be most effective (especially since you can tell it locales that are “ok” for you to receive). There are a couple of howtos online which have some instructions for this - I can help you if you run into problems. Since you’re on a dedicated machine, I could probably install the backport of a newer version on your machine for you (globally) if you want a global install rather than installing it into your user space.
As far as filtering 8 bit characters, I’m not actually too sure. Does adding the filter fail if you try to put those characters in? That might fail a “sanity check”. I’ve never actually done this myself, but from doing a quick google, it looks like (under normal circumstances), 8 bit characters can be inserted directly into a .procmailrc.
You could disable filters from the panel and then edit the .procmailrc by hand, but if you don’t understand procmail, this may not be a good solution.
I know it’s unfortunate that we don’t (yet) offer better builtin filtering solutions. However, I will repeat what I’ve probably said before - I really don’t recommend using the filters in the web panel to try and do much serious content-based filtering. They’re intended for fairly simple tasks, and generally work for those tasks. It’s kind of like using a sledgehammer where a scalpel is needed. Trying to match words and strings found in spam never really helps for long - usually the spammers adapt quickly, and you’re left with a big huge mess of filters that aren’t that efficient and don’t do a whole lot.