Justin -
I would be careful when making assumptions even for something clearly dated like your kabuto. Besides the date do you have any other evidence to suggest it was made "FOR" the fight that was then in progress? Remember that battles happen, are referred to by different names by the opposing sides and may only be known to history by the name applied by historians.
Keio 4 was an intercalary year which included an extra month. Like our leap year this occurred only occasionally to help adjust the calendar. It would have been natural for an armour-maker to include this (戊辰) in the date but would not mean it was meant for the conflict unless he said "made for so and so of Satsuma" or for some other group we now know was involved in the fight.
If we were to speculate it would most likely be someone associated with the Shogunate since they had the money and the connections. Keep in mind though that it was a mixed bag on both sides of the conflict and some units from both armies were using older arms. Also that many Daimyo chose to opt out of the whole thing, choosing instead to remain at home isolated from what was a political fight at the capital. Why not stay home and live as you always had, and just join up with whoever the winner turned out to be? Someone like this could easily have commissioned some armour while sitting safely on the sidelines.
Any idea where this was made? That would be the biggest clue and as always photos are appreciated,
-t