You can do some simple bass lines and chords on an Anglo, more so on 30+ keys than a 20-key box. For example, I do play 'Beaux of London City' with some parallel notes in bass and treble. It is just a bit tricky in places because the lower and higher octaves are not laid out in the same way!

I have never seen any system of tablature for free-reed instruments which can show anything other than a single melody line. If anyone has ideas, I would like to add chords and harmony lines to my concertina tablature format.

Also, providing a chord chart would be useful. You can get quite a variety of chords if you know where to find them!