To the extent that working memory is just like having a sheet of paper to one side where you can write things, we more or less already have that, though I agree it could be better integrated.

And faster, by many orders of magnitude! Modern PCs already have ram capacities on the order of tens of gigabytes. If we're talking about simply written words (as opposed to arbitrary drawings, the storage space requirements for which are somewhat trickier), that's not the equivalent of a page, or a book - it's on the order of a library. And they can read or overwrite the whole thing within a few seconds.

Yes, in principle you can do the same thing by hand. In practice, writing out even one full ram dump by hand would probably take longer than a current human lifetime.

That by itself would provide a speed-type superintelligence advantage, to the extent that flat (i.e. not associative to the same extent human memory is) memory is a limitation on our intelligence.