This answer is still a work in progress...
A Unix emulator named "Apple //e Emulator" by Randy Frank (aka "apple2e" or just "ap2e" in filenames) goes right back to 1990.
It used ProDOS order for both ProDOS and DOS 3.3 disk images!
Note: the UNIX files contain all the data on a DOS 3.3 5.25 disk.
However, the files are not in straight t0s0,t0s1,t0s2... order.
Instead the DOS 3.3 files are in PRODOS interleave format. So the
first 256 bytes are t0s0 but the second are t0se. This is exactly
the format you get by reading a DOS 3.3 disk with the PRODOS block
PRODOS UNIX files are just a block by block binary dump of the data
on the 5.25 disk.
It seems this emulator did now allow loading and saving arbitrary disk files yet, so it did not use the
.po file extensions.
The emulator was distributed with a pair of disk images for each disk operating system named as so:
s6d1 yes DOS 3.3 slot 6 drive 1 disk arch
s6d2 yes DOS 3.3 slot 6 drive 2 disk arch
PRODOS.IMAGE.D1 yes PRODOS slot 7 drive 1 disk arch
PRODOS.IMAGE.D2 yes PRODOS slot 7 drive 2 disk arch
Now it could be that there was an even earlier Apple 2 emulator and I don't yet know which emulator or tool first used the file extensions
.po for this format, or which emulator first used the DOS 3.3 sector ordering format.
UPDATE: I found the distro archive with the disk images and added the link. The DOS 3.3 images are in ProDOS format as confirmed by opening them with CiderPress. The ProDOS images both have a bunch of junk at the end. After truncating them to exactly 140K CiderPress confirms they are ProDOS in ProDOS order.