A very interesting concept - let me know if you end up dropping it, I may pick it up and finish it.
Here is one idea as to how this might work. Say you want to encourage specialization between shields or armor. You could set the initial tech investment for level 1 at a very high cost, and then have the following levels at a much cheaper rate. This would simulate the sweat work involved in "getting the basics", after which improvements come much easier.
In game terms, you would have to spend 2-3 times as many tech points to get level 1 in both armor and shields, than you would have to spend to get to level 10 in either shields or armor alone.
What do you think?