Attaching marks to ligatures, but not mark-to-ligature

In Noto, I keep beating my head against https://github.com/googlefonts/glyphsLib/issues/756. Basically, the issue is this:

* Someone designs a conjunct glyph of `a-somescript` and `b-somescript` and calls it `a_b-somescript`.
* It has a mark attachment anchor `bottom`.
* glyphsLib, being glyphsLib, assumes that because the glyph name is a combination of two other glyphs names, it's a Ligature glyph.
* Because it's a Ligature it ends up in the GDEF "ligature" glyph class.
* The mark feature writer looks at this glyph, notices it's a ligature, looks for "bottom_1", doesn't find it, and doesn't write anything.
* The mark doesn't attach.

I can get around it by setting all these glyphs manually to being bases (or alternatively by renaming all my anchors), but if I have a `bottom` anchor and some marks have a `_bottom` anchor, that means I want them to attach together. I *think* it should be fine to match these anchors and issue mark-to-base rules for these glyphs if no `_1` etc anchors are present (heck, even if they are), even if the base glyph is actually in the ligature glyph class definition.

Does this make sense to others?