I don't think it makes sense to do this on the OpenMPT side, because this means that every musician using the tool we're talking about here would have to enable this setting. It's not just OpenMPT but almost all trackers start counting channels at 1, so if another musician wanting to use this tool would like to use e.g. Schism Tracker instead of OpenMPT, the same feature would have to be implemented in Schism. or Milky. Or....

It would make more sense to let the user of the tool specify channels in the range 1-64 rather than 0-63 and the tool internally then should just subtract 1 when passing these channel indices to BASS.

I know. Therefore I asked this feature to be hidden, and not 0-63 by standard. i.e. 1-64 for everyone as standard (seen by anyone who starts OpenMPT normally, and 0-63 for special occurences (hidden feature) that I need (please see my email if you haven't already, can't talk about it here, why I need it).

Yes, the second solution I/we already had too. But could lead to bugs on the tool-developers side. We'll see.

But could lead to bugs on the tool-developers side. We'll see

The other solution would lead to bugs in OpenMPT and any other tracker where this needs feature to be implemented.
OpenMPT prints the channel number in many different GUI elements - off the top of my head: pattern editor channel headers, general tab channel headers, pattern editor status bar, Go To dialog - and I am 100% sure I am missing some GUI elements here, meaning that the feature would be half-broken if I implement it now. It is much easier for Marco in his tool since he can easily review any call to all BASS_* functions and check that the correct number is passed to BASS. Implementing the same in OpenMPT is much more error-prone because we cannot easily find out in advance in which code locations the channel number will be printed.

Ok, I'll ask Marco if he can do it in the tool, instead of this feature being implemented in OpenMPT.

As per Marco, this issue is resolved on his site.