don't be shy now-
sounds good. think worst case scenario.
from the start. if you were pressurizing the line you wouldn't have any gas backing into your lead, and the vent would be at vacuum all the time from the airflow to the lead. so, no problems flushing.
i would build a tree that had the fan at the top of it. use a fernco to couple. in the tree i would install a backflow valve commonly used for sump lines in case you did have a backup. you would imagine that would catch most waste in the event of backflow.
design the tree so it can be uncoupled at the bottom where it hooks to the lead. install a valve at the end of the lead under the couple for the tree. this would allow for isolation and removal if the need ever arose. a plastic 4" ball valve would probably be fine.
experimentation will dial the system in. the only real problem i foresee would be the vacuum at the house being so great that it would suck the traps dry. probably this wouldn't happen, but it would easily be solved with custom made, longer traps.
i would do it. the underground room could come in handy for all sorts of reasons.