Do note that UiPath is the sole entity that can say if something is or is not within license terms, below is just my opinion.
You do know that you’re basically asking how to break license terms, right?
That would defeat the purpose of having separate license types for each of them (especially taking into account that FO is cheaper).
I highly doubt that this could be done without weird setups.
FO by itself should run fine on a server machine (so a user can log in, run his processes, log out - as you described) from a technical perspective, maybe even license perspective as well (not a lawyer ). But making FO ( = Attended) work like BO ( = Unattended) is IMHO a no-go.