ShadowPurser,
I have just hunted through our manuals and have come up with the following: 737 SEP manual: If the FD are not present at the time of boarding the SCCM must check with the Engineer that the EL switch has been armed.
FCO 2533b: If it is intended to board passengers without FC being present, the SCCM is responsible for ensuring that the ground personnel are aware of the chain of command.
Earlier in that section it states that in the absence of the FC the SCCM is responsible for deciding if disembarkation or evacuation is necessary.
It is therefore not necessary to have an Eng or FC on the FD, however the emergency lights must be armed by suitably qualified personnel, i.e. an Eng or FC and the SCCM should remain in the forward door area and advise the CC that the FC are not onboard.