Not sure about the code, or what MLO is, but if the feeder is
non-metallic sheethed and would otherwise be exposed on a ceiling or
wall I would definately consider metal conduit instead. Just take the
NM sheathing off the length you need to put into the conduit. You
might need to run the feeder into a j-box with the conduit coming out
the other end, unless there is some other approved transition device,
from NM sheathed to metal conduit.
I know the NEC specifies that any exposed wiring in a basement, garage
or workshop (meaning not run through joists or studs behind sheetrock,
for example) needs to be BX or armored cable to prevent the conductors
from suffering any damage.
I would assume that the same principal applies to an exposed subpanel
feeder. If you're trying to save money by not hiring an electrician I
would recommend asking the old guys working behind the counter at the
electrical supply house in your town.
That, or pull the proper permit and have it inspected. They'll be sure
to let you know what's what.