Yeah I've cut soybean hay before... great stuff!!! I had to cut it with a standard drum mower and then rake it to get it dry, lost a lot more leaf, but the best way is with a mower conditioner. The soybean stems are kinda tough to dry and need to be crimped to split them open to dry out with the leaves. Like alfalfa, the nutrition is all in the leaf so you want to handle it just like alfalfa, which is to say, as little as possible. A haybine will take it off right above the ground and crimp the leaves, stem, pods, beans, and all and leave it ready to dry down well. Rake it in early morning when the dew is still on to prevent leaf loss. Let the windrows dry down til it's JUST dry enough to bale then roll em up quick (or square bale it if you're a glutton for punishment)
It is a little coarser than grass, alfalfa, or clover round bales and so it doesn't weather as well; netwrap or better yet bale bonnets, tarp, or barn storage is best.
It won't hurt the conditioner. The best stage to cut it is at about half pod fill, when the beans are in the milk stage and the plants are still good and green. They'll just flatten a bit during crimping. If you wait until the beans start to dry and the leaves start naturally senescing (dropping) and the pods start to turn, you lose a lot of nutrition with the leaves going and a lot of pods will split and drop their beans. The year I made soybean hay I had planted them as a rescue crop after the cotton had failed, because I needed hay and figured that if the beans actually made good grain I could buy hay, and if they didn't make a decent crop I could bale them. Plus, with the cotton herbicide in the ground, planting sorgo-sudax was an iffy proposition at best. I disked the fields flat and planted soybeans and rolled the field flat so I could either cut for hay or combine when the time came. They got about 3 feet tall or so, and then it started raining a good bit when the drought broke (that wiped out the cotton crop) and the johnsongrass and morningglory went nuts, and then the armyworms started moving in on it, and although it had beans it didn't look really promising or worth the expense of fighting the armyworms and then having h3ll combining it because of the johnsongrass and tie vines, so I just cut it for hay right quick. It turned out really well. The grass came on and I got a couple late cuts and had plenty of hay, so I fed a few soybean bales and the rest was regular hay. I had put them in bale bonnets (sleeves) and decided to feed the leftovers the following year just to get rid of them. The tie vines really screwed up the drydown and the johnsongrass made a mess of it too, and the beans were really a touch damper than I would have liked to bale them, and they were a two years old, but I figured, 'Well, I'll put them out there and they can eat them or lay on them, whatever!" and you know what?? Those cows would go SO crazy over that soybean hay that they would walk right over FRESH bahia hay I had just unrolled to eat those two year old weedy soybean bales!!! They stayed fat on them too!
Sounds like you've got a good proposition there... I'd run with it!! JMHO!! OL JR