xo_attr doesn't really apply to JSON
rather than that, you can do: {:name/encoded version/text version}

so:

snprintf(fmt, sizeof(fmt), "{:%s/%%ju/%%s}", name);
xo_emit(fmt, (uintmax_t) val, size, buf);

%1d %1d %1d became %1d %ld %ld (one one one, became one ell ell), was that indentional?

Any time the 'width' of the output is limited, you should use have an 'encoding' format version that is not limited

for example: xo_emit("{:name/%16s}", string);

should be: xo_emit("{:name/%s/%16s}", string, string);

so the xml/json version is not truncated (or padded), but the text version is

fixed width number

did you mean to add a literal dollar sign here?

you could tag the 'usec' as the units, but probably not worth it

%-%s is meant to have a * isn't it?

Are these %1d -> %ld conversion here intended?

Why not {T:LIMIT/%-20s} and similar here, as in line 1654 or vica-versa?