"Mount" a loopback device from SD card without root

I'd like to move Termux/Nix-on-droid on my SD card as I lack space on internal storage. Unfortunately, SD cards are formatted in extFat so they can't deal with stuff that are common on ext4 systems, notably symlinks. I was thinking to use for that a .img file formatted in ext4 using mkfs, and mount it somewhere. But since my device is not rooted (and I don't want to root it for various reasons) I can't mount the file, even via adb:

/sdcard $ mount DCIM/virtualfs test_mount/                                                                                                                                                              
mount: bad /etc/fstab: No such file or directory

Is there any trick I can do to do that? For instance using some fakechroot-like tools?