Do you mean that you want to transpose the notes just temporarily, i.e. not modify the pattern data? Because otherwise, if you want to modify the pattern data, you can already achieve this with Find&Replace:

• Note: Any
• Find in the whole song
• Only in channels: x to y (you might have to repeat this multiple times if drum channels are interspersed with melodic channels)
• Replace with: Note: Transpose...

Let's say there is a tune with 10 tracks. Each track is a separate instrument. 2 tracks are drums and 8 tracks are other instruments (bass, guitar, etc.).
You need to transpose all instruments except drums.

Yes, the suggested workflow can achieve exactly that. You run a Find&Replace for all channels but the drum channels. Am I missing something?

Prior to this, I believed that you could only find/replace a single (one) instrument per swap. And I always specified a new tool with each change. And so, until I changed all necessary instruments.... It never occurred to me to leave the window with the instrument name empty....
Thank you.
Still need a detailed OMPT manual....

Without doubt the OpenMPT manual is all but complete, but I think it would have been possible to find out how to do this (and also the Grow / Shrink pattens) request using what is already available in the manual, because both functionalities are described there. Find & Replace in particular is an extremely powerful tool, which can be used to do many things that people have requested as separate features, so maybe it would be good to add a list of "recipes" to the Find & Replace documentation to serve as examples what kind of things you can do with it.