Same as Marty here, what has helped in the past for me.....
If I can hear gut sounds and some manure is being passed, I pull all grain and only give soaked hay and soake alfalfa. Probiotics are a give in when giving antibiotics, bute or banamine. Electrolytes are a great thing also. Trailer rides, walking them in an area they may be a little nervous does wonders too. If they are passing manure, graze for 1/2 an hour ( if they are accustomed to grass) then pull and walk, let them rest and repeat every few hours.Grass, as well as alfalfa, is a natural laxative. I do this as well as the soaked hay.
I would keep them off all grain until stools are normal for a few days, then slowly start them back on their feed and add some water to it as well as some veggie oil to keep things lubed up. I have used Milk of Magnesia also. This was once again in a case where they were passing manure but it was firm and few. A full bottle for a 200-250 pound mini, I believe the average bottle is 12 oz.
The length of time this has been going on and the fact that banamine is being used for any length of time would prompt me to want them on some gastro guard/ulcer guard. I have also given enemas to see if I could stimulate the horse to want to push and help ease anything that may be at the very back end.
If her gut has no sound,I would be concerned about an impactions, and believe it or not, I have dealt with a horse that was passing very small amount of manure, essentially moving out what was in the tail end of her track, but still had an impaction, slowly but surely her gut sounds went down to zero. Another concern, depending if and where there is an impaction, is that there may be gas collecting In Her cecum and it may need to be tapped.
Is your vet sure it is gas colic? is there any reflux when your mare is being tubed? Reflux indicates nothing is moving down, no reflux indicates things are slowly filtering through. Does the manure that is being passed have a sheen to it? indicating the oil is working its way through? My biggest success with impaction issues, especially when I was nearing my wits end after administering bags of lactated ringers and round the clock walks, was to take the horse to a 24 hour large animal hospital and put the horse on a constants drip and have her monitored and given pain meds as needed. Having a cath put in and giving fluids at home when it is warm out is not a big deal if you have a great vet that is willing to work with you and they are comfortable with your level of competence. Doing it when it is near zero and snowy out, is another story.
Sometimes the horse needs to be given fluids to get them to perk up and get things moving. There is no shame in bringing a horse to a 24/7 facility and telling them the course of action you are taking is hydration and pain management, if that is what you can afford/are comfortable with, meaning surgery is not an option. Believe it or not, they do understand in most circumstances.