Thanks for the response!
This isn't a "please will you implement the following" post, just to be clear. If it were, I would have posted it in suggestions. I'm trying to get a sense of what you guys have in the pipeline for forge in the light of the 1.7 changes, so that I can know if there is any mileage in my implementing either mechanism, or whether it is a non-starter/ unnecessary.
I do think that it would be very much in Forge's favor if it allows a whole bunch of mods to continue working. We know that a lot of mod code is very poor quality (hard-coded ids being a pretty common occurrence) and that many will just die rather than be converted across. A solution that gets around that problem would buy a lot of brownie points with the modding community. I can also see why you want to be rid of ids once and for all of course.
As to the removing a mod issue, it isn't so much about whether blocks from removed mods turn to sand, it is about whether a removed mod will result in other blocks having different ids from the ids they had before. I'm proposing a solution that is robust to modified block ids.
I'm open to having a detailed discussion about either, and certainly open to doing the work to implement them. I realize that, as it stands, I'm just another person talking about modding minecraft, but if someone from the team wants to have a further discussion about either idea, I'm happy to try and prove the ideas to you.