>  - Have you reported this to any BIOS engineers to confirm the cause of
>    the bricking?
Not really. The girl who answer my phone from DELL side doesn't really
know what NVRAM is... But I have reproduced this issue many times. (for
finding the threshold.) I simply occupy nvrams by:
for i in `seq 20`; do head -c20480 /dev/urandom | efibootmgr --quiet --create --append-binary-args - ; done
>  - Have you got a dmesg from the machine before it broke?
No. But with a diagnostic kernel(without change the behavior of
efi. Simply print total size and remain size when I write something to
nvram. Provided by Lenny), I got following output:

[  254.811220] EFI: efi_query_variable_store, total size=131072., remain=22968.
[  254.978133] EFI: efi_query_variable_store, total size=131072., remain=21924.
[  255.016127] EFI: efi_query_variable_store, total size=131072., remain=21758.
[  255.183644] EFI: efi_query_variable_store, total size=131072., remain=20714.
[  255.209139] EFI: efi_query_variable_store, total size=131072., remain=20546.
[  255.376831] EFI: efi_query_variable_store, total size=131072., remain=19502.
[  255.414849] EFI: efi_query_variable_store, total size=131072., remain=19332.
[  255.500374] EFI: efi_query_variable_store, total size=131072., remain=18288.
[  255.538204] EFI: efi_query_variable_store, total size=131072., remain=18116.
[  255.623828] EFI: efi_query_variable_store, total size=131072., remain=17072.
[  255.662147] EFI: efi_query_variable_store, total size=131072., remain=16898.
[  255.829512] EFI: efi_query_variable_store, total size=131072., remain=15854.
[  255.868963] EFI: efi_query_variable_store, total size=131072., remain=15678.
[  256.036129] EFI: efi_query_variable_store, total size=131072., remain=14634.
[  256.062325] EFI: efi_query_variable_store, total size=131072., remain=14456.
[  256.229330] EFI: efi_query_variable_store, total size=131072., remain=13412.
[  256.270257] EFI: efi_query_variable_store, total size=131072., remain=13232.
[  256.437379] EFI: efi_query_variable_store, total size=131072., remain=12188.
[  256.477963] EFI: efi_query_variable_store, total size=131072., remain=12006.
[  256.644949] EFI: efi_query_variable_store, total size=131072., remain=10962.
[  256.672662] EFI: efi_query_variable_store, total size=131072., remain=10778.
[  256.840152] EFI: efi_query_variable_store, total size=131072., remain=9734.
[  256.881992] EFI: efi_query_variable_store, total size=131072., remain=9548.
[  256.967256] EFI: efi_query_variable_store, total size=131072., remain=8504.
[  257.008474] EFI: efi_query_variable_store, total size=131072., remain=8316.
[  257.093713] EFI: efi_query_variable_store, total size=131072., remain=7272.
[  257.138413] EFI: efi_query_variable_store, total size=131072., remain=7082.
[  257.305960] EFI: efi_query_variable_store, total size=131072., remain=6038.
[  257.348840] EFI: efi_query_variable_store, total size=131072., remain=5846.
[  275.674513] EFI: efi_query_variable_store, total size=131072., remain=17086.
[  275.717700] EFI: efi_query_variable_store, total size=131072., remain=16892.
[  275.803479] EFI: efi_query_variable_store, total size=131072., remain=15848.
[  275.846323] EFI: efi_query_variable_store, total size=131072., remain=15652.
[  275.931921] EFI: efi_query_variable_store, total size=131072., remain=14608.

When remain size greater than ~5kb it monotonously decrease. 
-- 
Best,
Madper Xie.