since there info
provides no way to determine the default path,
I suggest we adopt your solution #2 in this form: make a leading or
trailing or doubled colon expand into the default path. That's what I
did for tex/web2c/kpathsea many years ago and I haven't had any
complaints about it.
If you do the work, great. (I'll need you to sign a disclaimer.) I
won't be able to get to this until after the release, whenever that is :(.
Thanks,