I don't remember doing that back in the day, and don't see ND filters on bhphoto.com right now, just multigrade. Maybe you can use a less bright enlarging bulb, or a longer focal length lens with higher f-numbers that you'd use at greater height. Typical enlarging lenses are set up for 8x10 prints so if you're printing smaller, the extra focal length can help you anyway.

If you're having fun enlarging then hey, go for it, but it's a very retro, expensive process with nasty chemicals and stuff, that's hard to justify unless you're trying to make serious art prints with high-silver, fiber based paper and all that. Of course that's even more expensive and uses a ton of water. Next stop after that is probably platinum prints ($$$$$$$$). Otherwise, you can do really well with monochrome ink jet materials.