Back in the late 50s and early 60s they made all metal slow-cooker-deep-fryers. Remember them well, they were fantastic except they had no insert you could take out and clean and the cords were not detachable.
You could probably use your Nesco but to be onthe safe side, I would give the company a call and ask them.
It may be OK to do so rarely on occasion, however, things are NOT BUILT the way the used to be and I would be leary to this on a steady basis.
Best ask the manufacturer. Know this is not much help, but I would rather err onthe side of caution.
Have used just a plain heacy copper bottom pan to deepfry in when we travelled in a travel trailer (no room for larger gadgets) and it worked fine.
Today's heat coils may not as be as strobly made as those of 40 or more years ago, best make sure by asking
Nesco.
Grelo