I didn’t try with a MOX, I installed it in one of my TOs: New For M.2 NGFF NVME M-key/B Key SSD to Mini PCI-E Adapter Card M.2 B/M Key SSD | eBay (and as it doesn’t really fit in there it is a little bit inclined what doesn’t matter as long as the connectors sit tight; I installed a m.2 2280 in it).
Regarding the length just measure within your MOX’ enclosure if it fits - total length is 9,5cm (without connectors of the adapter). If it doesn’t fit, you can try to use a riser card in between.
It does work, yes:
root@turris:~# lsblk
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
mtdblock0 31:0 0 1M 0 disk
mtdblock1 31:1 0 7M 0 disk
mmcblk0 179:0 0 7.3G 0 disk
└─mmcblk0p1 179:1 0 7.3G 0 part /
mmcblk0boot0 179:8 0 4M 1 disk
mmcblk0boot1 179:16 0 4M 1 disk
nvme0n1 259:0 0 931.5G 0 disk
