This is good!

I have made one, that geotuscan creature I showed over on the Best Numberals thread, that thing has ligated numbers by way of a two level crossbar arrangement. Commercially available in about three months.

Yes you'd have loads more fun---design potential---with hanging figures. I tend to make uniheight lining structured fonts

j a m e s

I don’t remember ever seeing or hearing of numeral ligatures.
What did Goudy say? “The old guys stole our best ideas” …
The first one is from Palatino (that is, the TrueType version Apple ships with its software), the second one from Everson Mono – latter being a ‘true’ (as in ‘touching’) ligature:

These custom condensed numerals are for a friend's tarot deck project. I made the 20 ligature because the same enclosed space in the design must contain the numbers 0 through 21.

That 20 ligature is gorgeous. See! This is the perfect excuse to get your ligatures on!

Yeah buddaye! Russell, this is a fine idea and just what we need---an excuse to make more ligurinos. I'm getting my ligated numerals on right now. I love your ligated number forty-five. Agreeably sinewy, iIt gives me a perfect excuse to use another hyphenate :^)

j a m e s

Had to share this.
The lovely Basile from Tipo has numeric ligatures/ligated numerals. PDF linked from the current FontShop Newsletter.