Basically, nothing will goes wrong. You can always put out dynamo for 700C on a 20" folding bike for example. The only disadvantages might be: you will get extra resistance. Because you will get extra output at the same time.
For detailed explanation
SP hubs all follow German StVZO 6V3W regulation for power output. Therefore, for certain speed, the hub output will be at a correct range.
If you put PL-8 on 20” wheel, it creates more power at low speed. It will be around 20% more at 5km/h and 10% more at 10km/h. After 15km/h, its output will be only slightly higher than PL-8 on 26” wheel.
The disadvantage is your will get extra resistance. It’s relatively fair because you will get extra output. If you choose 6V3W for 20” hub for your 20” wheel, then you will not get extra resistance.