That was quick, thanks for the patch!

I have a couple of requests and comments from the user side:

• I think for the sake of brevity we can just go with Default Attribute instead of Default Attribute Name
• Right now it seems to me that once you set the default attribute, you cannot go back to a regular value as default, even when you remove the default attribute string. This should work differently. I'd say that either...
  • the default state is fully dependent on the string and is exactly used when it is not empty.
  • or there is an additional toggle for the default state. That would allow for a default attribute to be set even when the default state is a value.

I think we can safely go with the first option though. (Maybe that behaviour was your intention as well, but it doesn't seem to be working that way right now.)

Note:
Testing this makes me realize that we should probably add an indication in the modifier UI that an input attribute doesn't exist on the geometry. When a default is used you don't have the benefit of selecting the name from the UI list of available ones and you can't see when it's missing from the outside.
But I realize how this can be problematic when accessing attributes from geometry that is not passed in in the modifier stack, like with the object info node.