Why not give it a try? How much effort is it to drag the open end of the EFHW under the rafters and give it a go? I suspect that, if you are considering this, your other options are not better. Twenty watts won't be any danger of setting the world on fire, but FT8 is known for being a weak signal mode.
Half a century ago when I was in a firetrap off-campus apartment I had few options to get on the air. There was a red-green pair of wires coming into my apartment and going across the road to a pole and, as I couldn't afford a telephone anyway, I loaded them up with a homebrew tuner. The antenna worked, not well, but it worked and I was on the air. If half a loaf is better than nothing, I suppose the end slice on the loaf is OK if it's the best you can do.
James Clerk Maxwell is famous for his equations on wave theory. However there was one principle that never made it into his notebook. It is "RF gotta go somewhere". And it will. Let us know.
73 de Norm W1ITT