Oh, I don't know. It is designed for one-handed use, after all.
The real historical combat lance (not the tourney version) is not all that different from a spear except it is desiged to be used one-handed.
Of course, it would not be very effective in real foot-soldier combat as you could not hold it out for very long. But you could hold it up and lower it to charge, just as you would when mounted. It's a pretty silly image, though, isn't it?
Anyway, I'd allow it, but not for routine use. I'd say you could use it if, say, you got dismounted and still had a hold of your lance. No, it's not very realistic, but, hey, this is D&D ... what's more "heroic fantasy" than using your lance one-handed after falling off your horse?!?
I wouldn't mind, though, if the rules were changed and it was listed as a large weapon, but treated as medium when mounted.