AFAICT, the AS200 builtin Ethernet in my AS200 works fine with tlp in
-current.  I tested switching in and out of full-duplex mode, and it
works as expected.

Mine is slightly different:

tlp0 at pci0 dev 11 function 0: DECchip 21040 Ethernet, pass 2.4
tlp0: interrupting at isa irq 5
tlp0: Ethernet address 00:00:f8:23:5b:74
tlp0: 10baseT, 10baseT-FDX, 10base5, manual

compared to yours:

tlp0 at pci0 dev 11 function 0: DECchip 21040 Ethernet, pass 2.3
tlp0: interrupting at isa irq 5
tlp0: Ethernet address 08:00:2b:e7:b5:17
tlp0: 10baseT, 10baseT-FDX, 10base5, manual

However, I think it would be best if you could test yours with
-current before I spend any more cycles looking into this.