I have patched the included knoppmyth kernel with the forced patch for 2.4.21 and I get as far as make modules_install and get this:
===============================================
find kernel -path '*/pcmcia/*' -name '*.o' | xargs -i -r ln -sf ../{} pcmcia
if [ -r System.map ]; then /sbin/depmod -ae -F System.map 2.4.21-xfs; fi
depmod: *** Unresolved symbols in /lib/modules/2.4.21-xfs/kernel/drivers/char/drm/gamma.o
depmod: gamma_driver_irq_postinstall
depmod: gamma_driver_irq_preinstall
depmod: gamma_driver_irq_uninstall
depmod: *** Unresolved symbols in /lib/modules/2.4.21-xfs/kernel/drivers/net/forcedeth.o
depmod: free_netdev
depmod: pci_name
===============================================

Anyone have any ideas ?

My steps were:
make olcdonfig
(answering "m" for the single question of if I wanted the experimental nforce driver I just compiled into the kernel as a N/Y/m)
make dep
make bzImage
make modules
make modules_install

insmod gives me something similar:
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root@mythtv:/usr/src/kernel-source-2.4.21-xfs # insmod forcedeth
Using /lib/modules/2.4.21-xfs/kernel/drivers/net/forcedeth.o
/lib/modules/2.4.21-xfs/kernel/drivers/net/forcedeth.o: unresolved symbol free_netdev
/lib/modules/2.4.21-xfs/kernel/drivers/net/forcedeth.o: unresolved symbol pci_name
=================