You silly twisted boy, Guy!
If you only looked hard enough below the mass of wiring and piping near the rear of the battery tray, you'd notice the Thema manual gearbox has a dipstick, just like the Beta's one!
Pour the oil in here!
I found that using a length of tubing attached to a squeeze bottle is ideal for getting access, although it helps if you have an assistant to invert and squeeze the oil bottle, while one inserts one's right arm through the maze of wires and pipes, and holds the other end of the oil filler pipe in the dipstick hole.
One wonders why none of those forward-thinking Lancia engineers didn't just use a longer gearbox dipstick and tube like that of the Beta Volumex?
I'll look into this modification and let you know if it works.
Kindest regards,