vagfilm (there are so few grands in Portugal, that there is no second hand market - in fact there is: people ask 5k for a non functioning grand…). Decent second hand uprights are easier to find.
My idea was to keep harp and strings in place, just asking a technician to release the tension of the strings and dampen and secure them with felt on the back of the piano (soundboard). Optical sensors would be placed like in cybrid at about the level where the strings were previously.
Do you think this is doable?
I grew up in Europe, lived half of my adult life there and I understand the situation there. The two world wars and the much less wealth that Europe had about 100 years ago have caused the lack of grand pianos that you are experiencing.
What you describe is certainly doable. In fact I did not say it was not doable, just that it did not made business sense in the USA given the abundance of inexpensive grand pianos, and the additional difficulties with the upright. I suspect it does not make business sense in Europe either (especially because transportation costs there are higher, not to mention most people live in apartments which are not in ground level as in USA, further complicating the logistics of moving the heavy boxes). Business sense or not, you are asking a different question, i.e. a DIY, so let me get to it.
If you plan to keep the plate (or harp as some call it), the piano will be very heavy. I suspect both the seller and the buyer (yourself) live in an apartment not at ground level, and I suspect the instrument is too big for the elevator you have in both buildings (if you have an elevator at all). So it will be expensive just to move the instrument from source to destination. That can be true of a grand too, but you might negotiate a disassemble at the seller's apartment (which I suspect most people would decline, but still you can try), and a the useful-for-a-hybrid parts of a disassembled grand can be moved by one person or perhaps two, and fit in most European elevators. The plate/harp will still be too large and heavy, but you could pay porters to help you remove it from the grand (help is definitely needed) and then haul it away for disposal. So you have to consider the final cost (by the way one thing is what people ask and another is what the instruments eventually sell for). But I digress again.
Back to your question: yes that is doable. It is a bit tricky because to hold the sensors well you need to put "something" (e.g. extrusion bars such as https://us.misumi-ec.com/vona2/detail/110302684350/?ProductCode=HFS5-2040-500 ) and this something need to be attached somewhere, and that "somewhere" must not be in the way of the hammers. The most convenient "somewhere" would be exactly where the plate/harp is located, so the ideal solution would be to remove it. But that's really a lot of work, so I don't encourage it. Finding a different way to attach the bar holding the sensors should be possible in most uprights, besides the most space-constrained.
So if you want to do that, I think you can: just make sure to look at the inside of the instrument and imagine how to attach such bars (take in mind that the soundboard might not have the necessary strength, unless you reach the backposts, so see where they are and if they match your needs). Given the cost of moving, which I suspect is high, and the "cost" of your spare time, you have to decide if it is worth for you compared to buying (perhaps a second hand) NU1 or NV5.
vagfilm I abandoned the idea when I realized that an acoustic action is a CLACKety mechanism that would drive me to divorce (or homeless or both)… of course, I could better isolate the cabinet with foam or some other dampening material because the soundboard would not be necessary, but even then…
For the noise, only your wife and neighbor could say if that's too much, and you can give it a try at a store with said NU1 or NV5 which in my opinion would be making a similar amount of noise.