Well, that made me decide to try all the other pcnet_cs options (especially ones aimed at Socket products), and this is the one which worked:
module "pcnet_cs" opts "use_big_buf=0"
I notice the source already has a special case for a similar card:
... if ((manfid == MANFID_SOCKET) && (prodid == PRODID_SOCKET_LPE)) info->flags &= ~USE_BIG_BUF;
Anyway, I'm now up and running, and many thanks for pointing me in the right direction.