The new feature is accessed through the set_maptext() proc:
var/HudObject/h = hud_group.add(0, 0)
// 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.