I see nothing wrong with doing this as it's just a chain tensioner. I'm about to do this on a Shimano 105 di2 7150 12 speed rear derailleur as there is no shorter cage option. I will be using this as a 1x setup off road so don't need a long cage or huge cassette. The other advantage will be more chain tension due to the shorter cage so less chance of chain dropping over bumps.
Yeh its possible, its the outcome thats the concern, how good are you at making stuff ? you got all the tools to pull it off ? you can make your own cage plates, you can take from another compatable derailier.
Your only limited by your abilitys, not for the average Joe but folk can do all kinds of things, so if your that type then crack on and show us the results, make a thread thats far from the usual common tat of, "whats my bike worth" or "what bike is this" no pic etc an all manner of other daftness : )
Now go grind, file, sand, tap, mill, tig and show us what you can do, a step by step pics gallore is always insightful for the forums.