Meh. Wasted an hour trying to sort this out.
The answer happened to be buried in some Android code. The fact that Android is open source is a cool thing–but I shouldn’t have to dig through that source kit just to figure out the answer to something that could be better documented.
When declaring a <declare-styleable> <attr> tag, you supply a name for the new attribute for your custom class, and you provide a format. Well, I couldn’t find the valid values for the format attribute in the documentation. But I did find them here.
And those values?
It also appears from the source code that this field is optional, and I presume if it is left blank, either one of two things will happen: this will default as a resource reference, or the format is only used for type checking, and this can be any value. I dunno; I haven’t tried it.
On the off-chance someone knows the answer, could they leave it in the comments?