I have Harmony Assistant. It includes a built-in programming language that can be used for all kinds of things and I'm pretty confident that I could write a macro for it that could import lots of ABC files and convert them to the Myriad standard if we had to. As has been noted above, these could be made available as an online version, and the plug-in does support a marker showing which note is playing. There is an example at the Myriad Website. Of course, one disadvantage of this is that everyone would have to install the plugin.