I have a feeling this might be more useful in the Comments tab, where you can see the sample names in context. You can also edit them there by pressing Ctrl+Enter so you wouldn't have to go back and forth between the sample and comments tabs.

I tried and seems like it doesn't work. I mean, I get the edit sample cursor but I can't write or delete anything

Whoops, something broke there indeed (probably when direct instrument preview from comments tab was implemented). Thanks for reporting!

As a start, r20545 updates the font used in the sample and instrument lists in the comments tab to be the same as the comments font (so Courier New by default).

There are a few limitations: Label editing will switch back to the "normal" font. I could have used the same font there, but in that case Windows gets confused when measuring how big the input field will be, so some characters will be cut off. As this destroys the purpose of being able to carefully align those characters with surrounding ASCII art, I thought that it would not be very sensible to use the font there. Instead, the standard font is used but at least you get to see the entire sample name.

Updated test builds can be found within a few hours from https://builds.openmpt.org/builds/ - let me know how this works for you.

In addition to these improvements, the latest test builds (r21980 and later) now also have an option to paste a pre-formatted text into the sample list on the Comments Tab. This would allow you to design ASCII art in a proper text editor and then just paste the final result into OpenMPT. I think this is a lot more sensible than further modifying the sample editor, where you only see a single sample name anyway, so a ruler doesn't really help a lot with drawing.