It's not completely ridiculous. I had the same thoughts and I can see a couple ways of doing it, although maybe not practical.
One way you could do it would variable lifters that at one end of its range would absorb some amount of the cam's lift which would decrease maximum lift but would also have the effect of shortening the duration - the lost lift might be acceptable since you probably wouldn't be using the shorter duration at times when you need maximum cylinder filling and it could be compensated for somewhat by making the maximum lift of the cam a little higher than necessary normally. I think I've actually seen this described somewhere else.
Another way would be to have split cam lobes running on concentric shafts and the rocker or lifter would ride on both of them. One lobe could be for opening and the other for closing and as you move them apart, the duration increases.
Your idea also occurred to me. As long as the lobes don't overlap and there's some space between one valve closing and the next one opening on a cam, then you could retard the cam right before it starts to close and then advance it again right before the next one starts to open to increase duration and do the opposite to decrease duration. But, since duration is way over 180 degrees and thus 90 on the shaft, with 4 lobes on a shaft, there's no gaps to play with. It might work on a V4 motor though.
The other way is to just license some VTek tech from Honda.
One way you could do it would variable lifters that at one end of its range would absorb some amount of the cam's lift which would decrease maximum lift but would also have the effect of shortening the duration - the lost lift might be acceptable since you probably wouldn't be using the shorter duration at times when you need maximum cylinder filling and it could be compensated for somewhat by making the maximum lift of the cam a little higher than necessary normally. I think I've actually seen this described somewhere else.
Another way would be to have split cam lobes running on concentric shafts and the rocker or lifter would ride on both of them. One lobe could be for opening and the other for closing and as you move them apart, the duration increases.
Your idea also occurred to me. As long as the lobes don't overlap and there's some space between one valve closing and the next one opening on a cam, then you could retard the cam right before it starts to close and then advance it again right before the next one starts to open to increase duration and do the opposite to decrease duration. But, since duration is way over 180 degrees and thus 90 on the shaft, with 4 lobes on a shaft, there's no gaps to play with. It might work on a V4 motor though.
The other way is to just license some VTek tech from Honda.