Depending on the eprom (and if it is potted) there are cables you can clamp over it to reprogram it without removing it. The problem is knowing what to reprogram since every manufacturer would have a different design.

Most of the older machines I actually want to use as a laptop seem to still have NOS or 3rd party batteries for (Thinkpads at least). Would be cool to have a Powerbook 170 or Wallstreet with a good battery pack but if you don't keep them charged and used the batteries will fail anyway.