Well, this wouldn't work as easily with the ultra-cheap (<$40US) board from New Micros, the NMIY-0031, it might work quite well with any of the old boards that use a DIL-packaged MCU.

You could place a daughterboard on the MCU socket, which would allow you insert "things" between their MCU and your physical MCU, thereby allowing ISP, and the provision of a connector to an external and off-board prototyping area.

If your goal truly is to make the pins and signals of the "standard" 805x visible to the new learner, the DS89C4x0 series is not a particularly good choice, as it doesn't have a 12-clock mode. It's a one-clocker, which is what I like, but it's bus cycle at top speed is probably too fast for anything external that's not discrete or programmable logic. It does have a convenient way to program the internal FLASH, and it does have a means for modifying the internal code, and it does have a means for modifying the memory map such that you can map out some of the internal code space in favor of external code space, and it does allow the use of its internal 1 kB of xRAM as code space. It also has features allowing the external clock to be multiplied, and it does have a built-in watchdog and voltage monitor. All these are handy, but they don't teach anything about the "biblical" 805x core. You could force it to work in a relatively understandable way, perhaps by providing code snippets that manipulate the features, but it will never "look" like a "biblical" 805x. Frankly, if you did all that work, it would be of more benefit to the Dallas people than to you, or to your intended audience.

If you want this to be an education board, then you should probably focus on connectors specifically for test instruments, e.g. oscilloscope and logic analyzer. Maybe it would be a good thing to keep the board as inexpensive as possible, and bundle it with one of those popular PC-based USB oscilloscope/logic analyzers. That has the potential of teaching quite a lot, and the combined cost could, if you're careful, still stay under $300US. After all, if the NMIY-0031 board goes for <$40US, surely you can produce something suitable and adequate for under $100US. After all, the assembled board alone is of little educational value without the attendant instruments allowing the learner to observe what's going on.

RE