If this is possible, it would be via readRaw, modifying the example from system.bacnet.readRaw - Ignition User Manual 8.1 - Ignition Documentation like this:
from system.bacnet.enumerated import ObjectType
from system.bacnet.enumerated import PropertyIdentifier
ot = ObjectType(1234)
pid = PropertyIdentifier(5678)
where you have used Wireshark or whatever other means to figure out the type and property ids of the custom objects.
I don't know if the underlying BACnet library supports custom objects or not, so I think you'll be a pioneer.