I've finally managed to route the front panel controller board. It took a while longer because (shock!) I did pulled yet another 180° turn on the bus layout. I located a cheap(-ish) source for the wire-wrap female headers, which means I can go back to stacking the processor boards together without needing two separate DIN-41662 buses. Instead, the required signals (and only the required signals) travel from board to board via the female headers. This comes at a small cost to board estate, but simplifies a lot of other things, including the processor routing.