Those are very good questions and you're right we are dealing with assumptions here. Again just basing this on welding/brazing experience and in no way trying to argue it is indeed fact but it is my opinion only. Be it lead, tin, silver solder or bronze brazing filler the metal does indeed penetrate into porous cast iron but something like cast iron is not like wood or other materials where something can penetrate deeply, we are talking less than .001 of an inch UNLESS there are hairline cracks that either can or even can't be seen then all bets are off as far as depth of penetration! When heated to the flowing point and introduced onto a metal that is slightly hotter these filler metals can indeed wick down to depths much deeper than just surface contamination and while ABRASIVE blasting, not bead or shot blasting, will likely remove lead contamination it will not get it all out of tiny cracks that are likely to be in any cast iron part that has been subjected to many heating and cooling cycles. I think the bigger question here would be just how much lead exposure is a person willing to expose themselves and their families to? Do I think a thoroughly sandblasted pot would be 100% lead free? No I don't and in fact I would seriously doubt it but at the same time I doubt a person is going to be exposed to any significant lead levels and there "probably" would be little to no danger from doing this. The truth is we can banter this back and forth all day but every case is going to be dependent on many variables and we all are just guessing anyway, the question the OP or anyone else considering this needs to ask will be "is it worth even a tiny risk for such an insignificant item"? Personally I wouldn't even consider it but that's just me and at the same time I doubt anyone will get very "much" lead expose if they do decide to try and clean something like that unless they fail to clean it thoroughly and in that case they could well be setting themselves up for serious trouble.
What is thoroughly clean? By what method is thoroughly clean accomplished? How much lead exposure is someone willing to accept? Most of all, take a look at the insignificant item that has been contaminated and ask whether or not it's worth any risk at all?
Just a few things to think about