Are you working with firmware version 4.08.2016071801?
I've been trying at this, but even an unpatched infiniiVisionCore.dll from the firmware image is giving me issues. With CFF Explorer, you can see its header has the machine type set to MIPS R4000, yet the disassembled instructions only make sense if you treat it as ARM.
About version 4.08 - no, sorry, have no any versions older than 7.10 for my scope. Very interest to look at this update pack.
By the way, have anyone the oldest update files by these MSOX300T? Will be great if you can share older files here.
About dll.
Mmm.. Yes, I saw this strange thing with it (MIPS), but after editing of one sign byte in this .dll file, the IDA detects this dll properly and disassemble it successfully, as ARM. Say more, the lot of DLLs inside nk.bin are have sign "MIPS" but have a code absolutely not for MIPS MCU. I think, the WinCE core ignore this sign byte, maybe this byte was maded for us, for make disassembly impossible. It's my opinion only.
Now, because have no DSOXLAN, I have a fight with MagJack for LAN pcb - can't found it somewhere not far from me... Thinking, for the first probe, use the usual separated pair of LAN frontend: transformer pack + usual LAN connector.