The issue isn't how to use or read BCD, and actually this is hex 'cos these are 16 positions switches. The issue is how to multiplex several rotary switches - each of which has 4 bit lines. It can look a bit like a matrix keypad except that it will have multiple "keypresses". you can get round that by using loads of diodes to prevent crosstalk but someone might have come up with a smart solution.

Andrew