Erik,
From the details provided, I guess you want the switch act like a intelligent hub - passing/flooding traffic
within a vlan including BPDUs. I am not aware of any way to acheive this.
Having pair of ports in dedicated vlans, we can make the switch acts like a patch-panel but only for L3 devices.
E.g.,
Router1-----(1/1-vlan10)Switch(1/2-vlan10)------Router2
Router3-----(1/3-vlan20)Switch(1/4-vlan20)------Router4
Here, Router1 and Router2 act as they are connected back-to-back. Again CDP is not going to work.
Regards,
Yogesh