The older Time Crisis games had calibration on it the first time you start it up as well, and I'm pretty sure that a cursor was on at least the calibration screen. The Wii doesn't offer the same level of calibration that a Time Crisis game would.
On the Wii you can only set it as 'sensor bar above/below the screen', then with +/- 3 vertical levels. It's not a sufficient setup for true 'line of sight' aiming, especially if your TV is not in an optimal position.
Time Crisis on the other hand tells you to 'aim at the center of the screen', which does use precise line-of-sight for aiming. The GunCon3 in particular is using even more advanced technology for this due to the use of sensors.

So, if the 'sensor blocks' for the Guncon 3 have the same basic function as the Wii's sensor bar, it's feasible to have a mod to work with both of them. However the Wiimote will not perform like 'a regular lightgun', it'll be the same as usual.

The PlayStation.Blog has been having a short series on the GunCon 3. I don't know how many they are doing, but there are 2 posts so far and the newest one was just yesterday.
Part 1
Part 2

The 2nd one in particular goes into a little detail on how accurate the Guncon is with the new setup.

Also, since it hasn't been mentioned, the reason all these things need sensors is because traditional lightguns don't work on a good deal of HDTVs (if not all of them).