Two problems:

Problem #1) Is it possible to bind something to Alt-UP? I have tried M-UP and ALT-UP, both prevent mocp from running, with this error message:

FATAL_ERROR: Parse error in the keymap file line 83: bad key sequence

Problem #2) I configured a few "fast dirs":

go_to_fast_dir1 = M-1
go_to_fast_dir2 = M-2
go_to_fast_dir3 = M-3

FATAL_ERROR: Key M-1 is defined for volume_10 and go_to_fast_dir1

Yes indeed, but the line that binds M-1 to volume_10 is commented out! Even deleting the line altogether doesn't solve the problem.

So I assign something else to it:

volume_10 = !
volume_20 = @
volume_30 = #

Now I don't get that error message anymore, but M-1, M-2 and M-3 do nothing! They should be triggering the fast dirs.

I end up binding just plain numbers to the fast dirs...

go_to_fast_dir1 = 1
go_to_fast_dir2 = 2
go_to_fast_dir3 = 3

That works, but it's not what I wanted.

Looks like a bug to me.