Evap drain is right next to the heater core hookups, to the passenger side of them; sticks out that same oval-shaped hole in the firewall. It's just a short black plastic spout that is supposed to have a rubber 90 degree elbow on it, though the elbow is often missing.
I think fabbing a cabin filter mod would be pretty difficult, but not impossible. The tricky part I think would be making an airtight seal around it that would still let you service the filter as needed. About the only place I think you could sneak a filter in there would be between the blower motor and the evaporator core; the system "necks" down right there and might give you just enough room to swap a filter out without having to pull the dash, though access would still be pretty tight. The more I think about it, there's just not enough room for access...likely way more trouble than it's worth, IMO.
You wanna just keep debris out of the airstream, pull your wiper arms and cowl plastic, and cut a piece of window screen to cover the large holes in the cab openings that the cowl plastic sits on top of.
Richard