You can use the special "variables" input, which is the input the script variables block uses.
However you can't customize the default variable names, or min and max slots, but you can use metaprogramming.
Which you can then just edit the input like normal to choose default names, and min and max slots.
This is different from the "variables" input type, because it's literally just a variadic upvar, whereas the "variables" input type is a special type that can't be customized.
However it doesn't seem like the variables are even created, so you have to
manually create them before setting them.
Even if you don't try to set the variable, it will also not exist
So here's how I was able to get it to work.
Which runs this perfectly fine.
P.S.
I just love how we're immediately doing weird things as soon as snap 10.0 released.
Edit: I've now fully figured out how to use it, and I created a new for each block.
Another edit: apparently this was actually already possible in snap 8.