I personally think you will have a problem as big bale haylage is ok when there is a lot of stock to feed, but the relatively small amounts that three small ponies can consume without either becoming hyper or developing laminitis would be wasteful of a big bale. Once opened, the air gets in and as you say it will only last about 5 days (if your'e lucky), to re-seal you'd have to 'evacuate' the air or the degeneration would continue - easier said than done.
One other thing, purely personally I don't like the 'spike' method, you are puncturing the wrap and defeating the object of keeping the air out (ok for big round bales of hay). Far better IMO, to use a bale grab - although admittedly it's a lot more expensive and you need the tractor capable of handling it.
I have seen big bale haylage fed to equines - in a racing trainers stables where there were about 40 thoroughbreds that needed a high energy diet, they got through a big bale in about 2 days. It did come out of the bale in slices albeit large and was apportioned equally to each of the horses.
Sorry to be pessimistic, but perhaps you could share the bales with someone else for a price or (I take it you had the haylage made from your land) sell the complete bales to someone who could use them efficiently, then re-source a supply of small bales/packs of haylage that would not be wasted?
Good luck anyway, hope you find a good solution.