You can flag all command echos to be silent/hidden, so only players with access to "ulx hiddenecho" can see them (+ the player calling the command).

1) Making all command echos silent:
This can be done in various ways. The script below modifies ulx.fancyLogAdmin so command echos are always silent.
local fancyLogAdmin = ulx.fancyLogAdmin
function ulx.fancyLogAdmin( calling_ply, format, ... )
local formatType = type( format )
if formatType == "boolean" then
return fancyLogAdmin( calling_ply, true, ... )
end
if formatType == "string" then
return fancyLogAdmin( calling_ply, true, format, ... )
end
return fancyLogAdmin( calling_ply, format, ... )
end
I have attached the script to this post in addon format. You can add it to your server by extracting the .zip in the addons folder and restarting your server.
2) Give groups/players worthy of receiving command echos access to "ulx hiddenecho":
ulx groupallow operator "ulx hiddenecho"
ulx userallow Timmy "ulx hiddenecho"