There's another version of the Broadcom (B57) NDIS/ODI driver at (https://packetdriversdos.net/).
It dates itself from 2011 (110802), and reports version 15 (15.0.1), so quite a bit later than any of the B57.COM drivers looked at so far. It's 64,575 bytes, so a good 8-10 kbytes bigger than the earlier versions.
Upload speeds using it seem to float around 13 Megabytes/sec, so it looks to be an improvement on the versions I've tried so far, in that direction at least. Though downloads are still a miserable 1.2 Megabytes/sec - I am beginning to think this is a hardware problem with the machine.
Not a great deal of luck on the PCMCIA front. There are card services drivers available from IBM/Lenovo (https://support.lenovo.com/gb/en/downloads/migr-4kk4xu), and these do support (amongst others) the T43, but... I can't get much working with them.
Of the following cards:
DLink DFE670 (non-cardbus 10/100) - uses DFE670PD.COM, a normal DOS packet driver
DLink DFE690 (cardbus 10/100) - uses DLKRCB.COM, a normal DOS packet driver
3Com 3CXFE575CT (cardbus 10/100) - uses ELPC575.COM, an ODI/Netware driver
I can only get the DFE670 packet driver to detect the card. Both cardbus cards do not get detected by the drivers, which leads me to suspect that the card services are only enabling the interface in PCMCIA mode, and not in Cardbus mode (it is a Cardbus port). I've also got a couple of Xircom 10/100/modem cards (both cardbus and non-cardbus), but they're Type III thickness, which don't fit - this is a real shame as I've had good experiences with these in the past (uses the CBEPD.COM packet driver).
The DLink DFE670 gets a lousy 920 kbytes down and 800 kbytes up, so it's a fairly rubbish implementation. I think I tried that particular card on a number of machines in the past and it sucks equally on all of them.
My collection database and technical wiki:
https://www.target-earth.net