The new feature is accessed through the set_maptext() proc:
var/HudObject/h = hud_group.add(0, 0)
h.set_maptext("hello!")
// it takes three parameters so you can set the bounds of maptext:
h.set_maptext("hello, world!", 128, 32)
// you can also set these values in the HudGroup.add proc:
h = hud_group.add(0, 0, maptext = "hello, world!", maptext_width = 128)
By default the HudObject's maptext_width and height are both 32, so even if you don't change them you'll at least see something. The way text is positioned is slightly different so you can't only change calls to set_text() to set_maptext(). It'll work but it won't look exactly the same. HudObject.set_text() is still useful. You can easily create custom fonts and its performance is fine for small amounts of text or static text.
I also updated some of the demos to include alternate examples showing how to use maptext instead of the library's text functions.
I think I found a very minor bug, though, and I have a couple of suggestions as well. The bug is in the interface demo, within the demo-form. The "my game" default value cannot currently be erased through any means other than pressing delete. My instinct was to click to move to the end of the text and backspace, but backspace working the way delete works would be fine as well.
My suggestions are adding a menu that displays active party members to the party demo, and adding a HUD bar that isn't repeat-able to the health bar demo, or as an additional demo. I only suggest this because you've covered everything else HUD/interface-like related that I can think of, so this would make sense to me and make the library feel more complete; at least to me.