I’ll also add that I2P doesn’t have a dedicated TransProxy AFAIK. Any other applications besides web-browsing would need you to pick out a supporting tunnel transport type to be able to move a given program’s packets.
GitHub - rbif/transi2p: Transparent proxying for I2P and forwarding other addresses to Tor or clearnet. seems interesting for a very different reason. Its an implementation of the idea I proposed for this topic: Block all traffic except some hidden addresses - #9 by assd
The only use case I can think of is to create some foolproof intranet infrastructure where services are hosted using Tor/I2P or to create some kiosk mode - assuming your users are completely clueless.