I'm finally going to ask for help.
Any advice is appreciated.
This is a 21143-TD hooked up to a Level One LXT970 PHY chip, built on to the motherboard. As far as I can tell, everything has been properly configured in the 21143 CSRs and the MII registers. The MII registers appear to report that autonegotiation is complete and successfully selected 100Mbps half-duplex. CSRs seem to agree. DOS packet drivers work and autodetect 10 or 100Mb correctly.