Unfortunately, NO. The problem (and this bug report) is for Python-3 documentation, so trying to address it in Python-2 rather than in Python-3 does not make sense.

We seem to both understand and agree that there is no length limitation on b2a_base64() input, either recommended or enforced - contrary to what the current Python-3 documentation implies.

We understand that *if* the *output* of this function is intended for use in MIME (rather than X.509 or whatever else Base64 is good for), then the caller should do other things besides calling b2a_base64(), and in all likelihood the caller is already aware of that - after all, if he figured that he needs Base64 in his stuff, he probably knows something about what MIME standards say and require?. 

I repeat my original complaint: Python-3 documentation is buggy because it implies a restriction on the input that is not there. This reference should be removed from there because it confuses people. 
I've talked to those confused personally, so this is first-hand.

I refer you to the original msg253572 of this bug report.

If you want to write a MIME-in-Python tutorial, it is up to you - but b2a_base64() does not seem to be the right place for it.  
(And I'd rather see an X.509 tutorial if you're dead set on writing something besides strict plain b2a_base64() doc. :-)