The Starbound 1.2 update was great, and the addition of custom currency support looked to be very promising, but after extensive testing, it seems that it's impossible to make custom currencies visible in the player's inventory. This, unfortunately, severely limits the custom currency system - if the player cannot see how much of a custom currency they are carrying, then that currency loses almost all value. This issue was described in more detail, including steps taken to test the currency system, in another thread of mine. Head there if you wish to know how I tried to implement a new currency system. My suggestion for making the custom currency system more useful is this: make the player inventory (as configured in /interface/windowconfig/playerinventory.config) accept custom scripts. Currently, the inventory seems to be running off C++ scripts only, making it impossible for modders to add anything of value to the player inventory. Custom LUA scripts should be able to access the inventory's panes and overwrite values within them, using widget.setText() or similar functions, so that the custom currencies can be added and updated in the player inventory. Here's an image of what would be made possible if the player inventroy accepted custom LUA scripts (taken from the previosuly linked thread):
