Very gently, in a 'buy you a coffee' kind of way, this is a fool's errand.
That said, you can keep an eye out of Ebay for a discontinued
Shimano Nexave Di2 FH-C810 low power magneto hub.
ooh that is an interesting hub. I didn't know it existed, even briefly. I may have to hunt for one to add to my obscure hub collection. You could use it to charge a powerbank to power your other electronic devices. Of course a front dynohub will produce more power and charge quicker.