@fuzzweed.co.uk are you sticking with SAM style ports? ZXi proposes some standardisation around $6E3B and $6F3B for SAA (and $6C3B and $6D3B for SID). There’s a bunch of music players and demos written for eLeMeNtZX and MB03 that would be compatible that way.
sinclair.wiki.zxnet.co.uk/wiki/ZXI_sta…