Tectite have two hidden fields - one blank field that auto spammers will fill-in, and another that is pre-filled with data that must not be changed - obviously auto spam programs change that too...
IMHO this is very weird approach. May be it's only for "dummy" programs (which ones?..) working that way.
Actually, knowing how these field work, any script could be adjusted to provide the proper information there is required.
I.e. it's possible to adjust the script to not fill the field A and leave the field B as it is.
I think there is nothing difficult to provide the similar approach in miniBB. To the registration form, and to the message form it has, you may also add two random hidden fields (modifying templates). They could be even completely random, f.e. each day they would have different names (this could be a bit of tricky code, but still possible). Then in bb_plugins.php, there could be a very easy code, similar to what I provided above, which verifies these fields and stops the script, if something is wrong with them.
If it's difficult for you to code this out, I could put this task under my tasks lists and get back to it as soon as I have time. I could provide such kind of solution, but with no including it to the default core. That would be ideal, because being a coder with some annual experience, I honestly doubt on this method... but if it would work for somebody incl. you, why not.
Keep on :)