I've tried to find pictures illustrating handpferden in context. This is the best that I have found so far: http://www.susanna-m-carminati.ch/events.htm. One picture shows mounted personnel leading an extra horse through dressage movements. This is only of limited relevance, since yor grenadier battalion was a foot unit.
That an entire battalion was told off for this duty indicates either that that many men were needed or the battalion was in someone's doghouse. You should be able to find if the latter was the case. If the former, then one must ask where soo many horses would be needing holding, or if that was a term that we are taking too literally. It also seems odd at first glance that a grenadier battalion would be told off on such a duty.
Then it occured to me where I had seen mention of something similar in a French army. At Waterloo, one battalion of the Old Guard was initially stationed at Le Caillou. I think the staff had taken over the farm and imagine that certain critical vehicles would be there such as Napoleon's coach and the treasury. Does this make sense in your context?