I had a lami pony when I was a kid, we used to feed her dried stinging nettles instead of hay. They worked wonders as they have less nutritional value than hay but gave her something to much on. otherwise she was on a small amount of meadow hay. I think that to feed haylage or seed hay you would have to give them such a small amount that they would be very bored and hungry. Don't know if that helps.
Just had the vet out as suspect my 13hh welsh mountain was on the verge of having a lami attack at the w/e; he went a bit pottery on his feet. On his advise I have kept him on for a few days on hay (soaked to remove nutrients, some say for 12 hours but depends on the hay quality and the fatness of the pony I think). He does think I should feed him Happy Hoof as it is good quality low cal fibre. I've done a bit of reading around and the gist is to avoid letting the pony eat grass which is likely to have a high sugar content; that is spring grass (esp now if it rains it will 'spurt') so restrict grazing with a muzzle/ limiting turn out time/ limiting grazing area to small area. Apparently 'stressed grass' has higher sugar content so not sure if I would go for the last option as I would assume that bare paddocks are permanently 'stressed' but really not sure on that point?! Also, sugar levels in the grass drop overnight so it is best to let them graze overnight if limiting turnout (the later in the eve you turnout the better).
Avoid all sugary & molassed feeds and treats, so no Likits, Horselix or anything nice basically!!
Oh and finally if you suspect your pony may be having an attack bring him in, put him on a deep bed to prtect his soles and get the vet as he if it is serious he can give drugs that may prevent it developing into a full blown attack where the pedal bones rotate.
Think that about covers it!
Old hay that is of less nutritional value, preferably soaked.
Ensure he has a good mineral/vitamin supplement and don't starve him. Hope he recovers soon.
I turned him out last night at 9.30 with a muzzle on (which he was very cross about and tried to get off!) and he had a good old buck, fart and roll so is obviously feeling fine in himself now! Brought back in at 7am for yard / stable rest with some soaked hay to munch and stop his tummy rumbling! I would soak good quality hay for as long as poss if you suspect he may be on the verge of an attack, or get some older hay that he will only eat if he's hungry.