> 4 bitplane 640 pixel scanline to output a 320 pixel 256 colour mode

yeah, you are right with 320. Means no need for any soldering inside Amiga, can just plug into Video Connector like A2024 monitor, and one RP2040 is enough.

> I vaguely remember a device back in the day

me too! I think it was dedicated to Video processing? or maybe bundled with a scanner? and most likely required you to massage bits on computer side.

Edit: found it! https://bigbookofamigahardware.com/bboah/product.aspx?id=481 I think this https://bigbookofamigahardware.com/bboah/product.aspx?id=503 also worked in similar way.