Thanks for writing. I am not an expert on s/qmail and if i said something which is not true I prefer to that everyone will raise a flag and i will correct it asap.
Now, regarding s/qmail and qmail-spp functionality i usually use my own scripts/logic on different stages.
Just some examples:
1) i want to reject connections if HELO/EHLO is not RFC2821 and i want to do this as soon as possible ( connection/helo stage)
2) for some reasons (maybe better logging) i will temporary move the same on the [mail] stage or even when [rcpt] is presented.
3) maybe i don’t want to use RBL on connection because i want better logging (ex: who send to whom, etc)
4) maybe based on some other checks executed on earlier stages i want to tarpit / greylist / whatever before going further.

Those can be easily achieved in exim and even with qmail/qmail-spp. If you said that you can implement the same with s/qmail than i am very happy to hear and i will obviously change what i wrote months ago.

Btw, do you plan to include it in distros where anyway software is compiled when install ? (ex: Gentoo )