i thinks its possible, youjust have to remove any trace of it being a targa for it to look good and be worthwhile...... lots of custom work though, like custom metal work and welding, new door seals, new roof lining and triming....lots of work.... and this is all just so you can have a solid roof over your head which is debatably no better than having a targa roof......at least it would be different though
I'm sure it would be a lot easier to make solid Targas... that is, use fibreglass or something instead of glass (so you can't see through them anymore) and put roof lining inside. That way you can still remove them if you want, but otherwise it looks 'sort of' like a normal roof.
and dont forget... unless you're using carbon fibre for the hard top... it wont really make it much lighter either, unlike a factory slicktop.