The idea of private messaging is a relatively easy one to talk to people about. Adding cryptocurrencies into the mix - when, fairly or unfairly, news coverage about it is mostly about financial speculation, the "dark web" and illegal drugs - is a terrible idea if the idea behind Signal is broad uptake of a secure messaging app that doesn't have links to companies like Facebook.
Whether the settings are buried or not people will find out about this as time passes, and when they do it's going to create distrust. As others have suggested, if you want to do cryptocurrencies, why not create a separate "Signal Wallet" app or something? Although even that won't help the fact that it will tarnish the "Signal" brand in the eyes of many.