I think a two computer solution might be easier. If one can get early serial on the target machine, then a small romcc program could read commands from the serial port, execute them, and transmit back the results. (I think I even saw an email from someone that did this in pure assembler.)
Do you mean llshell?
Then, a specially modified emulator could run the original bios on a second machine, forwarding memory reads/writes and io reads/writes through the serial port to the target machine.
It should be possible to create a sort of emulator/redirector that executes most of the stock bios code in an emulator environment (say bochs), but redirects all in's, out's & memory accesses to the target hardware via serial. (llshell on the target can already do this part). Eventually, youd end up with a trace of commands that init the ram among other things.
Although one gotcha would be anything that's timing related. Memory/IO access proxied through the serial port would also be orders of magnatude slower. When the DRAM test completes, we'll all be dead :) Also, rom bios code memory references would have to be excluded as well as anything that could upset the serial port. What about handling stuff like cpu/mmu state transitions (ie real mode to protected mode to unreal mode, etc)? Cache? Perhaps the proxied memory access could be limited to just a certain range (if known).
If you use llshell, note that the current version doesnt support amd's 64bit registers/addressing yet. p4/32bit is ok.