Could it work?

Yes, but it would require:

• A redesigned memory controller capable of tiering RAM (which would be more complex).
• OS-level support for dynamically assigning memory usage based on speed (Operating systems and applications assume all RAM operates at the same speed).
• Applications/libraries optimized to take advantage of this tiering.

Right now, the easiest solution for fast, high-bandwidth RAM is just to solder all of it.