Keeping it short tonight since I've been terrible about sleeping a proper amount at night.

Shortly after posting last night, I remembered that I should be looking at the linker script for hints as to what it's expecting. Sure enough, up by the top there's something like:

__interrupt_vector_9   : { KEEP (*(__interrupt_vector_9 )) KEEP (*(__interrupt_vector_timer0_a1)) } > VECT9

So, of course I tested the human readable version

.macro SECTION_VECTOR vector
.section __interrupt_vector_\vector,"ax",@progbits
.endm

.global test_int
SECTION_VECTOR timer0_a1
.word test_int
.text
test_int:
    push r9
    mov #9, r9
    pop r9
    reti

Compiled it and did an objdump -S:

Disassembly of section __interrupt_vector_9:

0000fff0 <__interrupt_vector_9>:
    fff0:       2e c1           interrupt service routine at 0xc12e

0000c12e <test_int>:
    c12e:       09 12           push    r9              ;
    c130:       39 40 09 00     mov     #9,     r9      ;
    c134:       39 41           pop     r9              ;
    c136:       00 13           reti

So there you have it. Mystery solved (or at least, was a mystery to me.) They were nice enough to allow a tolower version of the vector name appended to __interrupt_vector_ be used to lookup the expected name. This was much easier than the weird preprocessor macro expansion I was trying to get working last night.