Shields are good because they provide a form of healing beyond the traditional direct or hot healing. I think if you want this kind of detail they need indicators that are specific to shields.
So shields should not be incorporated into health. The underlying health needs to be cured regardless. There are really just two health sizes: tanks & everyone else. Adjusting the max would just make it so you have no idea how to relate a deficit size to the heal size required to fix it for a particular character.
Some kind of bar indicator would be better I think. Or a color coded square based on shield size etc.
Another approach is to have shields show up like heals do at the moment. So if your heals show up as green or lighter colored on top of the health deficit, then perhaps shields can similarly show as an additional layer on top of both the heals and health bars.
As you say this does all depend on Blizzard providing the data needed though.
And yes, Pastamancer & Maia did a brilliant thing conceiving and implementing Grid.
I added a plugin for mainhand and offhand weapon buffs for Grid2. If you are able to use Grid2 pretty much as is and are willing to reset it from scratch when changes are made that need a reset then its a solution for your problem.
Note that the api does not work on a unit, it only works on the player's main and offhand weapons. It will only show up for yourself unless blizzard changes something in the future.
...weapon enchants, ... cannot get them to work with MissingBuffs...
Most statuses in Grid operate on the unit itself ("player", "raid1", "pet" etc.) Therefore this would need a plugin that checks your weapons for it to work in Grid / Grid2 etc. I am not aware of any that do this so you need to find a volunteer to make one.
...You mean the transparent health bar that shows how much health the unit will have after receiving a heal? Is it able to show the overheal?
I am curious why you care to see theoretical overheal. Actual overheal can only be determined after the fact when the unpredictable damage the boss did during the heal casts shown is taken into consideration. With other words, the "overheal" bar could be twice the health of the tank and yet the tank is already dead because it will not be enough to cover the damage the boss is about to do.
For me the usefulness lies in having it show who is getting heals at that moment, and do the heals top the person off. For example during aoe or right after some mass damage event.
Having the incoming heals be the same color as actual health but with more transparency makes it easy to differentiate. I am pretty sure you can do that in both Grid and Grid2.
...Except this information is only visible if output to the center icon...
This kind of thing is why Grid2 is a little different behind the covers. It is however still alpha in many respects so you are stuck with how Grid does it. The center icon is Grid code, the other icons are plugin code. It could be unified in Grid (I spent some time trying) but it is a lot of work and at that point you are really just writing Grid2.