I feel that when \u#### is used in the source, the XKB files should always use U#### notation. That way, the user can preserve a code point exactly as it was intended.

The issue that 〈 〉 are expanded to wrong/outdated(?) brackets may not be so simple but it'd indeed be a matter for the XKB people. I had to read up on the matter:
• Different fields use different brackets.
• The U+2329/232A code points actually decompose to U+3008/3009!
• These are the Asian CJK punctuation brackets, so they're commonly used.
• Physics happily uses U+3008/3009 in bra-ket notation it seems?
• Thus I've used U+2329/232A in my math dead key table. I'll change that.
• Maths and physics should rightly use U+27E8/27E9 as you said?!
• On the other hand, there are other mathematical signs that are often substituted.
• In sum, I'm unsure as to whether there is a simple right answer here.

But by preserving Unicode values, users can get the one they're after.

As far as I can see, there are far more than two sets of brackets in Unicode, all in all:
https://en.wiktionary.org/wiki/%E2%9F%A8_%E2%9F%A9
https://en.wiktionary.org/wiki/%E3%80%88_%E3%80%89

It turns out that I was wrong in outputting leftanglebracket. In the documentation of keysymdef.h it says that for some characters (like this one) the corresponding Unicode point is vague and shouldn't be used, but I did. I updated the parsing, so it should be fixed in the next version.

I do not think it is needed to always explicitly output U+XXXX, as it is harder to read. In this case it was also not needed to differentiate, as the output was wrong.