Max,
An antenna as you described would make a good wireless
'dog fence' on 160 meters, but that's about it, I'm afraid.
Just slightly 'better' on 80 meters, and on 40 meters, just
very slightly 'better' than that. It will become 'almost-
an-antenna' on bands higher than 40 meters where it's length
is more than just a fraction of a wave length, but don't expect
much performance, NVIS or not. That's for transmitting. On
the other hand, it'll make a very quiet recieving antenna for
bands where it's length is more than a small fraction of a
wave length.
From what you say (no supports of any kind for an antenna)
I think you are going to be 'hurting' for an HF antenna, other
than a vertical of some kind. Is there any way you could
attach a wire doublet of some length to the eaves or peak of
your roof? Doesn't necessarily have to be of any particular
lengths or symetrical shape Longer is better), although that
would peobably help. Any height is better than no height,
as you'll be finding out...
'Doc