The variations in powder charges for black powder firearms can be huge. For example, you might use one 50 grain pellet for practice and plinking, and double the charge to 100 grains or even triple the charge to 150 grains for hunting. I don't know of any loads using smokeless powder where you can double or triple the charge. If you are developing a plinking load in a rifle, you usually use an entirely different powder.
Second, I think that most people who use pellets in a ML are hunters. They arn't trying to get the absolute most accurate load out of their rifles. They probably don't try let's say, 75 grains, then 78 grains, then 82 grains or whatever and tune the load until they get the best possible group. Black powder hunting is usually a fairly short range affair and you don't need to get the last fraction of an inch in accuracy. I would guess that someone who does demand that kind of accuracy out of a ML uses loose powder and not pellets. Most rifle handloaders on the other hand do try to get max performance out of their loads and spend at least some time trying different combinations to get something that provides adequate velocity and accuracy with the bullet they have chosen.
In order for this to work, you would have to use specific, standardized loads. This actually might work in some cases (providing that the primer will ignite the charge). I know that bullseye shooters use some pretty standard loads for .38 wadcutters, or in .45 ACP.
