pfSense

I want to be able to capture packets on the base physical interface for a physical interface that only has assigned VLAN interfaces, no assignment on the physical interface itself.
For example, the setup I have is somewhat like this

igc2: <unassigned>
igc2.10: guest
igc2.20: staff
igc2.30: contractors
igc2.40: admins
igc2.4090: mgmt

1. So I can do a promiscuous mode capture on the whole interface and pick up packets for all VLANs (even ones that I haven't defined) with their 802.1Q header
2. So I can check to see if the device on that interface is sending untagged packets or packets to VLANs I don't expect while trying to debug an issue I'm facing

The base interface still exists and is up, so I can run tcpdump -i igc2 from a terminal and that will capture what I want but it's a pain to save it to a dump file and download it to a computer with Wireshark.
It's not possible to do this from the Web UI since igc2 has no assignment. If I do assign the interface it seems to mess up the VLAN interfaces receiving packets correctly so having a temporary or bogus assignment is not a good option.

I can imagine several other situations where a specific interface (physical or virtual/logical) has no assignment but is active and a packet capture may be desired.