I've re-checked your proposals about "hidden fields protection" and I think they are still worth of doubt :)
You wrote that
It is use by the Tectite Forms we use for registration of downloads, and we have had ZERO spam since we have been using it (5 years or so). [...] It works 100% in application (from our 5 years of use). In comparison we have a lot of spammers hitting the forum... even with 'Captcha' enabled, etc.
I'm not sure I know what your app is about, but here we truly may talk about different things. Spammers register on forum because they think they URL is visible and clickable to public. The most important is this hyperlink's moment - they do not understand it's clickable for logged members only and has no effect on SEO... but in your app, there could be a case when registration doesn't require a password, signature or anything else to visible to public. Users just register to download a file and their profile is hidden - this may be a completely
different case. In this case, spammers do not attack you because they do not see the sense of it.
I also have some few specific projects on the web which require registration, and which are not even protected by Captcha (just email validation) - so what, in a few years there are no bulk registrations, because profiles are not visible to public, and they do not contain anything SEO-specific which is worth for spammers... it's a different case.
But... the form "protected" by hidden fields could be tricked easily in many ways. I could even program a script in PHP which will bypass this "protection". It's a very primitive approach, created for similarly primitive hackers, I suppose - which could just take a ready software and apply it to some website by few clicks, with no having understanding of how and what works there...
So for me it seems the add-on which cleans up registrations within some time period, is still the best for miniBB in this aspect.