I have a customer who has an old floor with this kind of finish, and she's asked me for some cleaner for it. To my knowledge any traditional cleaner that has water in it will cloud it. Am I wrong? I tried some Bona cleaner on a sample of it and the sheen changed, although I was able to buff it again.

What should I be recommending? How about regular maintenance?

Wax finished floors were quite common from the manufacterers up till the mid 80's with Bruce being the leader in using the penetrating oil and wax finishing systems. The correct maintenence for these floors is to clean with a dust mop or broom, buff periodically and strip and rewax periodically. Bruce still makes the proper maintenence products for these types of finished floors.
http://floorcleaners.net/ssi/osb/itemdetails.cfm?ID=127

Note: Since the site here is not selling these products, I hope Ken will not mind me listing another commercial site.