I understand your frustration, I feel your pain. The first time I tried to buy batteries for a set of tools that only cost about $120 (w/ 2 batteries) and was told it would cost over $100 just to rebuild the batteries, I couldn't believe it. That was a Craftsman drill with a light, charger, and 2 batteries. I actually kind of liked it. I tossed it and bought another set on sale for about $20 more than the batteries for the old one.
I have since found that I can do far more things, far better with a Festool drill than I ever could with either or both of those drills. Each new drill also gets better - that is the reality of the technology involved. You didn't mention if you have the right angle or eccentric chucks but they make a huge difference in the utility of a Festool drill.
I don't think Li-ion technology is typically, if ever, backward compatible. What I mean is that you shouldn't take a machine that was designed to use NiCD or NiMH batteries and simply bolt on a Li-ion battery.
Unless I'm completely confused, Li-ion batteries require a feedback loop with the usage mechanism and the charging source. There are different charging cycle technologies but they all require feedback from the battery. There are also requirements on the usage side (the tool) and the simplest that I know of is to stop the tool when the battery drains to a certain extent.
It isn't practical to produce Li-ion batteries for a NiCD tool. The worst that could happen is fire, Li-ion batteries have been known to (sort of) spontaneously flame. It's actually predictable but appears spontaneous and mysterious... The more likely scenario is that you will have a single or very few cycle battery.
Tom