am not familiar with FOSSIL so cannot say what the relationship between FOSSIL and a COM port / COM port driver will be.
Fossil is a standrd that developed in the BBS world to deal with multi port serial cards. It's a shim layer between the OS COM ports and multi-line BBS software. As long as one can identify the port and interrupt for a serial port the fossil driver can connect with it and provide communications via int14 for up to 254 serial ports.
When running DOS software, like WWIV, under OS/2 one has the option of loading a dos level fossil driver in the VDM. SIO provides an os/2 native and virtualized version of the X00 fossil driver that lives outside of the VDM space in the the OS memory space - it's just a more resource efficient way of doing things.
Not that I have to worry about resources in this case, but still, old habits.

Anyway, while I would _prefer_ to be able to use the built in port, with sio to save resources - there's other options. I can load the dos level version of X00 and depend on the more modern drivers getting COM1 into the VDM. I could also just abandon COM1 in favor of a pci mutliport card on a pcie bridge.
SIO v2 introduced it's pci support, so in theory I could get it to interact with the native com port on it's PCI home instead of at its dummy isa address, but i'd have to find it in the hardware tables and figure out how to teach sio where it's at.
I can make sio not attempt to service com1, and have validated that I can do hardware com1 with 6 vmodems via fossil - with all 7 acessable from the VDM. I do need to get com1 on fossil for doors and network packet handling though.
My rough plan is to use the built in port, along with 6 vmodems from sio, and a local only node to eat up my original 8 node WWIV 4.x license.