Can an Oracle ConnectX-3 be flashed with stock firmware?

I have four ConnectX-3 dual port 40G cards, currently configured as IB ports. The firmware is identified as v 2.11.1280 but the PSID is ORC1090120019 … can these be flashed with a newer stock firmware that will allow mlxconfig to switch them to Ethernet ports, without having to struggle with Oracle support?

No. For OEM card you have to contact OEM vendor.

However, you might try to use port_type_array parameter for mlx4_core and see if it helps

Some examples, here -

Also check older Mellanox OFED user manuals, version 4.9 or newer that still support ConnectX-3 card