Accepting with a bit of rambling thoughts and some UI tweaks below.

I agree with putting this in a separate panel. I thought it might make sense to make it a subpanel of the existing attribute panel, but since those are "outputs" it doesn't really make sense.
Another option is renaming "Output Attributes" to "Attributes" and adding two subpanels to it: "Outputs" and "Used Named Attributes" or something.
That doesn't feel any better to me though.

I wonder if the new panel should have a poll function so that it only shows when named attributes are used.

It would be nice, actually the best solution to the UI here IMO, but unfortunately I don't think it's possible currently, the panel poll function takes the panel type rather than the panel, so it doesn't know which modifier it's from.
Maybe there is some way to add the panel's modifier to the context used by the poll function, but that didn't seem doable when I looked at it.

I tried to make the panel look a bit more consistent:

I don't think that's perfect, but I think it fits better.
The contrast is low on the "Read"/"Write"/"Remove" words, but I think separating them visually is really helpful, not sure if there's a better way to do that.
Aligning to the property split 0.4 factor also helps when there are other buttons.

I put the attribute name on the right side to align with the output attributes panel. I also removed the quotes since I don't think they're necessary after the other changes and removing them makes it a bit cleaner.

/* #uiLayoutRowWithHeading doesn't seem to work in this case. */
uiLayout *split = uiLayoutSplit(layout, 0.4f, false);

std::stringstream ss;
Vector<std::string> usages;
if ((usage & NamedAttributeUsage::Read) != NamedAttributeUsage::None) {
  usages.append(TIP_("Read"));
}
if ((usage & NamedAttributeUsage::Write) != NamedAttributeUsage::None) {
  usages.append(TIP_("Write"));
}
if ((usage & NamedAttributeUsage::Remove) != NamedAttributeUsage::None) {
  usages.append(TIP_("Remove"));
}
for (const int i : usages.index_range()) {
  ss << usages[i];
  if (i < usages.size() - 1) {
    ss << ", ";
  }
}

uiLayout *row = uiLayoutRow(split, false);
uiLayoutSetAlignment(row, UI_LAYOUT_ALIGN_RIGHT);
uiLayoutSetActive(row, false);
uiItemL(row, ss.str().c_str(), ICON_NONE);

row = uiLayoutRow(split, false);
uiItemL(row, attribute_name.c_str(), ICON_NONE);