Thank you for your response, that makes sense why I cannot directly use VLAN 0. The packets that I sniffed from the router I need to emulate indeed has the VLAN tag set to 0 and a priority tag set. This is a blurb I found elsewhere on the internet:
Of the 4096 possible VIDs, a VID of 0 is used to identify priority frames
VLAN 0 packets should carry an 802.1p priority but otherwise should be handled as untagged packets
So I suppose my question is, am I doing something wrong with my bridge filter to apply a priority tag to an untagged packet, or is the Mikrotik router not applying the priority tag because there is no vlan tag?