I have no idea if this is the way it works...
- that's my honest answer too...!
Erm..., if it's not too much trouble, imagining you've already taken the tablet apart, could you post a photo of the double fpc cable and connectors where they connect to on the tablet (and the LCD end if you think it would help), and it's a good idea to say the model tablet/LCD, or failing the model tablet any identifying reference on the tablet main board and ideally the LCD or something similar if they have any, if you want to, otherwise it's just guessing rather than actually trying to give as informed an opinion as possible. For one reason, some-one here might spot a device on the board that will help to narrow down how to go about it.
Also, I don't want to bombard you with useless questions, but does the/do the fpc cables have numbers at each side next to the connector or anywhere along the cable, e.g. "1 40"? Is it possible to count the strips (or connector pins) to know how many contacts/cables are actually used?
To be honest with you, I hate to sound negative, I don't want to at all as your project sounds really fun and worthwhile, and surely is worth trying, but also it sounds very hard to do without locating the actual hardwares' datasheets/product info., etc. and in that way being able to put together what you want to with some idea of how this stuff works (before damaging the LCD, for example, which could happen if I tried to do this by guessing and connecting things anyway), unless you already know most of how these things work.
No idea if a touch to USB board exists, seems odd there aren't a few appearing in seach engine results as it is a fairly obvious need/handy device for more than a few people.
By weld I'm understanding you mean solder!
Sorry to ask, just "brainstorming" with you here - but do you reckon the "a board which is already built and programed" for plug-and-play may not be a definite option as perhaps this board and LCD were designed in-house to be very application specific for Android devices or for a board just for whoever manufactures the tablet?
I'm sorry I can't offer any practical advice, but do recommend posting a photo or two. Your idea described at the end could work, as I think you are clearly more versed in programming and these devices than I am (I only learnt the day before yesterday what apt-get means, and that was a big leap forward for me). By all means, if you know how to analyse the signals and translate them to appropriate signals for USB, then as you suggested get the connectors and solder them to a bit of pcb, but if it were me I think I'd do a bit more research first into the make and model of LCD and try to get hold of the manufacturer's datasheet.
Sorry, but don't forget the photos if you can upload a couple.
To end on a positive note, awwww - your screen is capacitative, mine is resistive, yours is supposedly better and more durable than mine