It sounds like your only option may be soaked down high fibre nuts as its a teeth & choking problem you probabley can't get away with the chaff hay replacers as it will just ball up.
Allen & Page do a fast fibre soaks in 60 seconds but you would have to check with them if it can be fed as hay replacer. I have used it to bulk out ponies feed as she has problems with choking but she is out on good grass 24/7 so doesn't need much at the mo. Also used Meadow Graze from Chestnut Horse feeds soaked down.